I severly doubt the vBulletin developers are going to help you with varnish, that is a technical performance decision which has nothing to do with the vbulletin software itself.
Anyway, here are some links concerning varnish and vb4:
http://vbtechsupport.com/290/
https://www.vbulletin.com/forum/show...in-share-yours
As a fair warning though: Varnsih configuration is a +++++ for a vbulletin board, since varnish does not know or care about whether the user is logged in or not. So you might end up showing your guests internal forums if you messed up your varnish rules. Or an issue we had, varnish doesn't care about whether your guest is a) a human non member, b) a member who just logged out or c) a searchbot.
Unfortunately vbulletin does care. b) and c) do not have a register button in their forumview, so if varnish uses their view as cache base neither will your guests have one.
Concerning your error i do not have an idea. Sorry.