I just found some good information on Vice City Stories from pspupdates.qj.net:

You'll be playing as Victor Vance - Lance's brother.

Victor, and his brother Lance, will be playing a big part in Vice City Stories. He's seen in the opening scene of GTA: Vice City, as he get killed by a SWAT team.

The game takes place in 1984 - two years before GTA: Vice City

 Victor is a 28 year-old US Marine

Players can now take on the water of Vice City With jet ski's!

The animations are more life-like and is a ''step-up from previous titles in the series''

 A multiplayer feature similar to the PSP version of LCS is in the game VCS is ''much, much bigger than Vice City''

Some landmarks are half-built or yet to be refurbished to their state circa 1986

There's new or altered locations and buildings

All new vehicles and weapons You can fly helicopters!

Radio stations from Vice City make a return

New weather effects that could include hurricanes

You can swim in the game!

Bikes handle better than before

There's also no more seamless exterior-to-interior gameplay as seen in LCS (There'll be a slight loading time)

A new addition in Vice City is the chunder-wheel, which you can ride in first-person view!

Binoculars is in the game

LCS is ''rookie first-gen PSP title compared to VCS''

Draw distance is great and is said to be better than Vice City on PS2

Trip skips make a return (If you die or get busted a taxi will take you back to the location where you received the mission from)

 New animations which give both the lead characters and pedestrians a whole new degree of humanity

 Far richer color palette that brings sunsets to all new levels of beauty, and realism

Increased density of pedestrians, cars and objects

Reduction in clumping (where groups of the same model type appear together)

Far more interiors that its predecessors

So far there's no word on character and vehicle customization, as well as wall climbing.

This game looks quite interesting and I can't wait for it to come out. It's an interesting choose on main characters and I like the fact that they are bringing back swimming. I don't see the big deal in water skis but it's new and it's not making the game bad so who cares? I would like to see what this one will be like:

New animations which give both the lead characters and pedestrians a whole new degree of humanity

It might be like people grocery shopping and actually having a purpose and parking cars places and going into buildings. I don't know though.

I also like to see that radio stations are back from Vice City and I look forward to listening to that chat station that I can't remember the name of. I hope there is more than 3 sketches to it this time.

Well anyway I can't wait for this game and I hope it's as good as Liberty City Stories which was my favourite Grand Theft Auto game.