global.php include problem

[Hammon]

I'm tryng to build a portal based on a cms that integrates with Vbulletin. On this purpose I include global.php of vbulletin in my cms pages as suggested in alot of threads.

It fails: Fatal error: Call to a member function on a non-object in /forum/includes/functions.php on line 1041
which is a call to the db function:

$user = $vbulletin->db->query_first("
SELECT " ....

I suppose the problem is that either cms and vbulletin uses a class named $db so when I try to use code from the vbulletin framework it doesn't work.
Is there any solution in order to use both cms and vbulletin framework thogether (db class) ?
THanks in advance for your help: being not a good coder myself need help :P