Gotenks isn't anywhere near Majin Vegeta is base. Yamcha and Krillin confidence was probably about SSJ Gotenks being able to take Buu. Shin and Gohan also survived a battle against Buu, are they stronger than Majin Vegeta now? Also, Buu is closer to Vegeta than to Goku. A suppressed SSJ3 Goku clowned Buu, who couldn't one shot Vegeta.
Was a different multiplier to SSJ Gotenks ever implied? SSJ Gotenks was >= Suppresses SSJ3 Goku, and Goku clearly wasn't +50x stronger than Majin Vegeta.