Problem I had was that:
XP default workgroup is WORKGROUP
Vista default workgroup is MSHOME
What I didn't know, but do now from experience, is change the XP to MSHOME; OR, Change the Vista machine's default workgroup to WORKGROUP. Either way will work, as long as both computers belong to the same workgroup. You could call the shared network anything, as long as they matched, and follow Windows' guidelines for naming conventions.