These are the common ones to use. The actual url may vary for you depending on your location. The four (packages, patches, extra and testing) are all you really need. If you enable more repos with those same four words in them, you will end up with duplicates all over the place when you list. Get's confusing as heck, and is a waste of time and bandwidth when updating your sources.
The many other repos that aren't vectorlinux are Slackware or other Slackware based repos. They have packages that are known to work with VL. But it is possible they won't.
I usually only have VL repos marked. Unless I can't find a package I need and HAVE to have it.