Glad to hear you are looking forward to Hustle Kings on Vita
The trophy lists for both games are the same. You'll be able to keep track of your trophies specifically on the Vita or overall PSN trophies using the Trophies app on the Vita. There's also an activity tab which means you'll be able to see when your friends earn trophies too.
There won't be a discount for owners of the PS3 version, but some of the Vita pre-order packs come with a voucher for money off downloadable games which will be valid for Hustle Kings.
We have made a lot of improvements to Hustle Kings for Vita along with various Vita exclusive features so it's not just a second copy of the same game you will be getting, it's almost a whole new game.