This can be found in the
threadbit template. Although modifying it would not give you the result you want.
When the link is clicked it goes to
member.php?u=123 not
member.php?find=lastposter&t=123
Code:
<phrase 1="member.php?$session[sessionurl]find=lastposter&t=$thread[threadid]" 2="$thread[lastposter]">$vbphrase[by_x]</phrase>
This is just used to find the last posters ID and without it you would not be able to link to the members profile.
you could use
$thread[lastposter] to retrieve the name of the last poster but the user id would not be fetched outside of that phrase.