[Christos Teriakis]

Just a "dirty" solution which means to manually edit the memberlist.php (repeating it on any vB upgrade). With a text editor (like Notepad) open the file memberlist.php. On line 375 you'll see:

$condition .= ' AND posts >= ' . $vbulletin->options['memberlistposts'];

just above enter:

$timecondition = time() - (365 * 24 * 60 *60);

then replace the line:

$condition .= ' AND posts >= ' . $vbulletin->options['memberlistposts'];

with:

$condition .= ' AND posts >= ' . $vbulletin->options['memberlistposts'] .' AND lastactivity >= $timecondition';

If you want you can change lastactivity with lastvisit or lastpost. This restriction will limit the memberlist to members who have an activity during last 365 days.
Finally upload memberlist.php to your vb installation.

Chris