First thing, check to see if you've created a shared folder in Vista. If there's nothing to find, you can't map to it. And, the built in "shared documents" has never allowed anyone to "map" to it. It's just plain available to see on the network.
Now, on the Vista machine, use the network setup wizard to create a small home/office network. If you have a router, use the "other" type of internet access option so it leads you through finding it properly. Near the end, it will ask you if you want to turn on file and printer sharing. Answer yes. In WinXP, there was a bug where it didn't get turned on correctly even if you thought you had the network settings manually set properly. Vista may well be the same way... somehow, it doesn't get it right during install. Once that is done, you should be able to map to a shared folder.