Is it actually said in the Japanese show or Manga that Vegeta gets a power boost from Majin? Because in the Funi dub all I remember him saying was that he'd felt he lost his edge since he started a family and cared for them, and that Majin simply eliminated those cares and brought him back to the mindset of being the ruthless warrior he was before.
Either way, I feel he definitely achieved SS2 on his own prior to that. He reached his ascended Super Saiyan form within a year of training in the time chamber without any prior knowledge of there even being a higher form of SS, so I have to imagine in the 7 years that passed in which he knew about SS2 he'd have strived for it and gotten there on his own. Nothing about his demeanor suggested he wouldn't have achieved it. Why would he have entered the tournament and seemed so confident knowing that Gohan is SS2, and likely Goku as well, if he hadn't also reached that level? Why would he look down on Gohan for getting weaker if he wasn't at least as strong as SS2 Gohan? You don't make fun of someone for having a crappy car when all you have is a bicycle.
If we're gonna talk about cheating Goku and Gohan had their fair share of help. Goku was nowhere near Vegeta's level until he had otherworldly training from King Kai, which gave him the Spirit Bomb and Kaio-Ken. He also achieved SS2 and 3 from otherworldly training, and when it came time to use SS3 with his living body he couldn't handle it. Gohan had power literally handed to him without having to lift a finger twice. First by Guru, then by Old Kai. He also had a lot of help with his training. As the future timeline tells us he only reached SS2 because he had Goku to hold his hand through the process.