A belated welcome aboard, Willy

Either game will work on Vista, though you should install someplace other than the "Program Files" folder for the easiest time with mods. If you want to play U-Boats, you need to get the Gold Edition or the regular game plus the add-on. It would be easiest just to get the Gold Edition and you would have the disk instead of downloading the add-on. Most mods are going to be based on version 1.5 (better known as SH4 + UBoat missions add-on).
The Uboat missions add-on isn't free (pretty close though @ $10 for the download version in the US), so you'd be ahead financially to go ahead and purchase the Gold Edition.