[jimp]

I was looking for the same thing, and was beginning to write a hack for it. Instead, I found the solution could be implemented with a phrase override.

Change pmreceived to:

Dear $touserinfo[username],

You have received a new private message at $vboptions[bbtitle] from $fromusername. Please click here to log in and read it:
$vboptions[bburl]/private.php

All the best,
$vboptions[bbtitle]

_______________
Subject: {$this->pmtext[title]}

{$this->pmtext[message]}

(Tested with vB 3.5.x)

I hope this helps!