Add a new plugin:
Product: vBulletin
Hook Location: member_profileblock_profilefieldbit
Name: Show Profile Field as link
Execution Order: 5
PHP Code:
if($profilefield['profilefieldid'] == X){
$profilefield['value'] = "<a href=\"{$profilefield['value']}\">{$profilefield['value']}</a>";
}
Change X to your own profiled field ID
Plugin is active: yes
this can bring some security issues depending on what the user sets the field to