The Juno I was a four-stage American space launch vehicle, used to launch lightweight payloads into low Earth orbit. The launch vehicle was used between January 1958 to December 1959.
Wikipedia's Vanguard_1; Mission says:
On 17 March 1958, the three-stage launch vehicle placed Vanguard into a 654 x 3969 km, 134.27 minute elliptical orbit inclined at 34.25°.
This is confirmed by the video 1957-1959 IBM 704 Computer - Vanguard Satellite Program "Science in Space" (Burroughs Datatron) NASA (alternate version of the video here) and screenshot below.
Question: What was the first successful two-stage-to-orbit launch by the US? What was the first attempt if different?
Screenshot of illustration of Vanguard 1 3rd stage within 2nd stage, from "1957-1959 IBM 704 Computer - Vanguard Satellite Program 'Science in Space' (Burroughs Datatron) NASA" link