May 15 of this year will mark the 100th anniversary of the founding of the city of Las Vegas. But it was 50 years ago that something much more important happened: Louis Prima and Keely Smith put Las Vegas on the map.

“Vegas ’58” (Concord 2266), the new album by Ms. Smith, who is appearing at Feinstein’s at the Regency for the next two weeks, is a lively celebration of the later milestone. In fact, the city centennial is hardly worth celebrating at all, since nothing of note happened in Las Vegas prior to the emergence of Louis and Keely.

Many big stars had already played Las Vegas, most notably Frank Sinatra as he was approaching the low point of his “nosedive” period. But before 1955, Las Vegas was just another stop on the tour. In fact, it was strictly the sticks, a place you played while you waited for an offer from a larger venue like New York’s Copacabana and Basin Street East.

Thanks to Prima, Ms. Smith, and saxophonist Sam Butera, Las Vegas became not only a destination but a place with a style and a sound all its own. Their move to Las Vegas marked not only a turning point for the city, but for Prima and his then-wife, Ms. Smith, as well.

Prima had an acute sense of jazz not just as a sound but as the spirit of a scene. He grew up in New Orleans, “the land of dreams” (as he sang about it in “Basin Street Blues”). As a young man in the early 1930s, the trumpeter, vocalist, and showman had been the first to put New York’s Swing Street on the map.

When Guy Lombardo enticed Prima to leave Louisiana and lead a band at the Famous Door on New York’s 52nd Street, Swing Street suddenly became the place everyone wanted to be. Prima thus had almost as much to do with launching the swing era – and putting a face on the new music – as Benny Goodman (whose best-remembered flagwaver, “Sing, Sing, Sing,” was in fact Prima’s composition).

Prima led one of the more popular big bands of the war era, but by the early 1950s, like his old friend Sinatra, he was in need of a new direction. As shrewd as he was talented, he undoubtedly knew he already had part of his future in Keely Smith.

Dorothy Jacqueline Keely Smith began as a child performer in the early 1940s (she admits to 1930 as her year of birth). By 1947 she was singing with local bands around her native Norfolk, Va. That year, she took a family trip with her brother Piggy and stepfather to Atlantic City, where they happened to catch a band she had never heard of before, Louis Prima and His Orchestra.

“The house was full of people watching this guy up there doing all this crazy stuff,” Ms. Smith recently told the Sun. “He was mesmerizing. You couldn’t take your eyes off him.” For months after the show, she kept thinking about him; she bought all the records of his that she could and committed them to memory. Back in Virginia, Ms. Smith convinced the owner of the club where she worked to bring in Prima’s band.

By a further coincidence, Prima was in need of a girl singer. Lily Ann Carol, who toured with the band during the war, had left to get married, and her immediate replacement was too nervous to go on. Ms. Smith sang informally with the Prima big band on several nights. When the Prima entourage left Norfolk – she still remembers the date – August 6, 1948 – Ms. Smith was with them. (“My aunt thought that I’d left home with Louis Armstrong,” she told me.)

Ms. Smith cut her first recording with the band shortly after the end of the musicians’ strike in 1949, “Five Foot Two (Eyes of Blue)” on RCA Victor – the first of many older songs they would jazz up over the years. Tony Bennett remembers Louis and Keely knocking them out when he shared a bill with them at the New York Paramount in 1951. But by 1953 – the year that Smith became the fourth Mrs. Prima – they could no longer get enough work to sustain the full band, and were picking up gigs where they could.

That’s when Prima accepted an offer from his old friend Bill Miller, one of many East Coast talent coordinators who had relocated to Vegas, to open in the lounge at the Hotel Sahara at the end of 1954. According to most accounts, this was an act of charity on Miller’s part, but the late Miller told Bruce Fessier of the Palm Springs Desert Sun he didn’t view it as such. He wanted a big name to open the lounge (eventually renamed “The Casbar”) and still regarded Prima as a headliner.

The show very nearly didn’t go on. Prima’s friend, the equally extravagant bandleader-vocalist Cab Calloway, was also appearing at the Sahara. Calloway, however, had to decline Prima’s invitation to attend his opening because all of Vegas was rigidly segregated. Prima was infuriated at such prejudice and refused to play. Even when the show finally started, the Prima and Smith combination was not an immediate success.

The Chief, however, knew exactly what they were missing, and he sent home to New Orleans for a saxophonist who had worked in a club owned by his older brother, Leon Prima. This was Sam Butera, a robust tenorist in the Illinois Jacquet tradition; Prima dubbed their backup band “The Witnesses,” in a dual reference to Christianity and crime.

The Prima-Smith-Butera-Witnesses combination opened at the very end of 1954, and they perfected their act over the next few months. By that time they were the talk of the music business and the act to catch in Vegas. Their remarkable book of material, including a series of intricate medleys – most famously the combination of “Just a Gigolo” and “I Ain’t Got Nobody” – was developed collectively, with Prima taking the leading role.

“We never knew what tunes Louis was going to call,” said Ms. Smith. “He would start with ‘When You’re Smiling’ and end with ‘When the Saints Go Marching In’ but it was anyone’s guess what he would call in between. And you know the band never used a page of music? They had all those charts memorized.”

According to Mr. Bennett, who was equally close to both Prima and Sinatra, the Rat Pack was directly inspired by Prima and Ms. Smith. “Louis absolutely created that whole Vegas scene,” he told the Sun. “I must have seen his show a thousand times, with Keely, then later Gia Maione [Prima’s last wife and Ms. Smith’s eventual replacement] and Sam Butera and the Witnesses. It was so wonderful, so spontaneous. I remember that Frank Sinatra and Sammy Davis used to come into the Casbar, and it was all that they could do just to absorb what was happening. I know Frank was thinking ‘Now it’s up to me to create something like this of my own!'”

The stage rapport between Mr. and Mrs. Prima evolved organically. At first Ms. Smith didn’t know what to do with herself on stage, so she did nothing. And the less she did, the funnier it was. Their act was a brilliant juxtaposition of maximalism and a minimalism: Prima would be practically climbing the walls and swinging from the light fixtures. She just stood there with a stone-faced expression.

The new group, however, didn’t make any records until April 1956. “The producers from Capitol Records came out from Hollywood and they wanted to record us. Louis said, ‘Fine, but you have to give Keely her own contract to make her own albums, too.’ But they didn’t want me and refused. So Louis just waited.” Finally, the company resigned themselves to the fact that to get Louis and Keely they were going to be forced to make solo records with Ms. Smith as well.

Though her first few singles (among them “Rock-A-Doodle-Doo”) were in the doo-wop mold, the Capitol producers quickly realized that she was a superior singer of the Great American Songbook. She made three Capitol albums that have been reissued several times, “Politely” with Billy May and “Swingin’ Pretty” and “I Wish You Love” with Nelson Riddle. The latter yielded a hit single in its title track, a certain translation of a French song by Charles Trenet, which has been Ms. Smith’s signature song ever since.

As supportive as Prima was in the couple’s early days in Vegas, Ms. Smith feels that their enormous success changed him. The two began to fall out shortly after they were ensconced in one of the larger showrooms, and they divorced professionally and personally. “I felt like I was nothing without him. I did almost nothing but look after our two little girls for a whole year after we split up,” she said. “Then Dinah Shore told me to get off my butt and come and sing on her show.”

Ms. Smith worked steadily throughout the 1960s, 1970s, 1980s, and 1990s, as a mainstream pop and occasional jazz vocalist-headliner, with only occasional references to Prima (who died after a long illness in 1978) in her show. And there was a long stretch when her fans in New York were denied the chance to see her. But she returned to the city in 2001, when her first new album in 15 years was released, “Keely Sings Sinatra” (Concord 4943).

It was at that point that the singer began re-incorporating the spirit of Louis and Keely back into her solo act. “Now I even sing some of Louis’s songs,” she said of the new album, which was recorded live at Feinstein’s. “He was a very quiet man offstage. I never realized how much fun he was having.”