Vegeta made it pretty clear he had no time to improve and was capable of fusing with a Goku that whilst stronger than him, was far more fatigued, so most likely not. His performance against Janemba could easily be down to Janemba toying with him.
I don't think so. Boo Saga established that he was pretty well capable of keeping up with Goku with all forms up until SSJ3. Vegeta's battle against Janemba was only slightly successful on the initial attack. So the element of surprise was working in his favor, best case scenario imo.
I don't see it any more or less practical than how long he lasted against Kid Boo's smackdown.
I think it's natural for any character to be haxed in anime/movies, considering the fights have to be longer than in the manga,
so the weaklings have to fare better.