1. All Vegeta says here is Buu will become weaker. And he's right.1. Chapter: 507 (DBZ 313), P7.3-6
Context: Vegeta grabs Boo's pod and wonder what will happens if he pulls it out
Vegeta: "This'll be fun...Will you turn back into this fatso? Or will you become that scrawny guy? ...Either way, it seems that your power will doubtlessly fall below what is it now."
2. Chapter: 510 (DBZ 316), P12.6
Vegeta: "The truth is that you were right [that I'd be killed by Boo]. That Boo is stronger than I imagined...And so are you, Kakarot..."
2. When Vegeta first saw Kid Buu, he thought he could defeat him.