Yes, it appears that Lothar Sieber was the first man to vertically launch in a rocket, the Bachem Ba 349 Mark 23 prototype anti-bomber craft he was test-piloting for the Luftwaffe on March 1, 1945. Unfortunately for him, later phases of its test flight did not go well, and he ended up dying in the crash.
There were three recorded successful manned test flights after that, but I can't find any reference to the names of those pilots. Presumably the first one of those would be the guy fitting your critera. It seems likely that information wasn't recorded (perhaps for security reasons at the time). However, its also possible there might be claimants after the war ended.
After the test flights, they ordered a production run, and were all set to launch the first operational flights on Hitler's birthday when the US army overran the operational launch site near Stuttgart. So it looks like it was a few years where those three test pilots (assuming they survived their other Luftwaffe duties) were the only living humans to have performed this feat.
The only online source I've found with good detailed information about the later stages of the project (after that fatal test flight) referenced the book Projekt Natter, Last of the Wonder Weapons, by Brett Gooden