Pretty much this.Goku was fighting for fun. Vegeta was fighting for someone else.
If Goku was actually pissed, it'd be no contest.
It wasn't like Goku had anything on the line in his first fight with Beerus. However, Vegeta was in a similar situation as to what Goku was on Namek against Frieza when he turned SSJ. So it doesn't really bother me now that I think about it. Should have made Vegeta go SSJ3 though