There is a wonderful add-on for vBulletin that offers this feature called "VBGooglemap Member Edition". As long as this is a legal (registered) copy of vBulletin - it can be downloaded from vBulletin.org by the owners/administrators of this site. It's quick/easy (both admin and user) and a great (low-resource) addition to any vB site. Another one is called "Vbulletin World Map System", and it's a mapping add-on that is automatic and based on log-on user IP's; where VBGoogleMap ME is user-generated and 100% by choice.
Here is a overview screen-cap of each from one of my vB sites (both can be zoomed, etc. by the user):
World Map System:
GoogleMap ME:
HTH (to the Administration of the site; feel free to contact me with any questions),