Reeve,
Thanks for the link. I also found a version via Google search at:
http://www.sartori.at/index.php?n=pages/vbext
I was thinking the same thing as you were for having one master vB, the problem will be how many sites you have (I think). Because to have it reference off a master vB it would be easier if all the tables for each site are in the same database. That could me 1,300 tables if you run 10 sites.
LDAP is probably the best way to go and I'll check out the link you posted to the mod here.
Issue I see:
1. If you create an account there will need to be a serive that updates the LDAP with new users as you pointed out. This also means you will want each record in the LDAP unique for each user so during the registration process you'll have to check the LDAP (all sites vb_user basically) to ensure the username is not taken.