Just to post the fix here in case someone else comes across this thread (I already helped you over on vb.com):
If you look in the navbar, you should find this line:
<phrase 1="$bbuserinfo[username]" 2="member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase>Try passing your field as phrase 1:
<phrase 1="$bbuserinfo[field5]" 2="member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase>