The idea, of course, is that the Vaseline reduces friction, friction causes blisters, so no friction equals no blisters. But I can't see any advantage to putting it either on your feet or on your socks, except as perhaps a short-term emergency measure. For one thing, I don't think it really works as a prophylactic against blisters, and it can cause complication such as what I mentioned above. It will ruin socks -- you'd need to wash them 10 times to get the gunk out. And if Vaseline gets into your boots it will absolutely ruin them by softening the leather and possibly causing the cemented midsole to de-laminate.
In short, the best solution is make sure your boots fit, then wear proper liner socks. Forget the Vaseline, except for...oh, never mind.