I'm not sure how you are executing the member_customfields plugin, since in 3.7 member_customfields is missing.
EDIT: I found this thread at vb.com:
http://www.vbulletin.com/forum/proje...?issueid=23995
Until such time as vBulletin 3.7 GOLD is released, my profile fields feature is unsupported. A lot of changes were made in 3.7, so I will release a new version of this mod at such time. The 3.7 version of this mod is a bit smarter/faster, has even more compatibility with other hacks, and is much easier to integrate should the need arise.