Been on a Villa forum and got talking to a local. Here's what he says if you're coming from the M1, M6 route. I've PM'd him back for alternate southern parking locations if possible, (for the M40 travellers). Will post if I get more info. Please feel free to add any other Villa parking information here.
If you have a program from the pompey game then there's a page that has a map and a list of safe secure car parks. It also said that parking restrictions are very tight around the area.
I parked near the King Edward pub before and that was Ok. Definately do not park at the leisure centre (where all the coaches go i think) as it takes about an hour to get out. I am just going pick a place from the map on the Watford site and get their early so me and my mates can have a drink beforehand in town.