I have no idea... but maybe I can work something out.
Both can be resisted by some way and we have no evidence that either is more powerful than the other. However, I read on the Veritaserum page that Occlumency is needed to block its effects (which was considerably harder for Harry than resisting Imperius) and even Barty Crouch Jr who taught Imperius couldn’t resist Veritaserum.
I’m going to go with Veritaserum “overwriting” Imperius and the person telling the truth, but it’s not impossible that they’d just block each other out and the person wouldn’t say anything... or say both at the same time and nobody would understand him.
(I was trying to do it scientifically and include all possibilities.)
