I agree that Brock's push was ridiculous and shouldn't have happened that fast.
I don't know Vince so I don't know what got him to do that. My best explanation is that Vince saw something in Brock and gave him the ball to run with.
But I've been a Brock fan since his RAW debut and despite the poor buyrates, he was in very good matches with Rock, Undertaker (HIAC), and Angle that I enjoyed very much so.