I personally wouldnt use Vbulletin to manage LDAP entries. LDAP is much bigger than VB and capable of doing much more. It would be better doing it the other way (VB feeds off LDAP).
To answer your original question..... yes you could use LDAP to manage multiple boards. In your case, you would need to import the plugin into every Vbulletin board. If you do this, users would seemlessly (spelling?) be able to login to multiple Vbulletin boards. If I had multiple boards, I would disable their ability to change their e-mail address, password etc. within VBulletin aswell. Always get them to change it in LDAP because it is the mother ship!
If you read my first post in my LDAP modification thread I've explained what the script does. It may be clearer then what you could do (dont wanna re-type it all here

). If your not clear, let me know and I'll try and explain better.
https://vborg.vbsupport.ru/showthread.php?t=145188