See whatever things are displayed in postbit_legacy like Joindate, Posts, Location and so on... notice that they are displayed in <div> tag. So, all you need to do is to make it
<div class="postinfo">
For example:-
HTML Code:
<div>$vbphrase[join_date]: $post[joindate]</div>
will become
HTML Code:
<div class="postinfo">$vbphrase[join_date]: $post[joindate]</div>
Thats it.