You can use the US account to buy things only and play on the other account. That's what I do. It's a lot cheaper that way. You can still use the good Vita deals on the UK account.
I prefer sticking with EU though. Often it's worse, but it has benefits such better Vita deals and being easier to get prepaid cards
And seeing as how i want all trophies on the same account..i don't wanna use two for different devices