From my experience you will not be able to install your game if it does not match the region of your registered PS account on your Vita's memory card. I downloaded an HK version of FFVII and tried to install it on my Vita with a memory card registered to my NA account and it won't install.

I don't know about PS1 games, but i did buy manhunt from the US store and it worked fine

They will work on your PS3, PS Vita i have no experience with. I even called the PlayStation customer service in the US to ask if it's not illegal or something like that! Happily it wasn't. They said they can't guarantee the software working though, but i never had a problem with my purchases (Resident Evil 2, Resident Evil 3, The Last Guy and Flower)

I'm in the Netherlands and usually buy from the EU store. You need US PSN credit by the way, unless you have a creditcard.

PS3 not a problem just switch to any account will able to play any games that is install in your HDD same for any region game disc for any account
Vita don't work cause the memory card will be locked to the account that you activated unless you willing to buy another memory card use another account contents on it