i had a situation similar to this but in my case it was regarding ASSIGNING CATEGORIES to items,
the problem in my case (couldnt see any categories to assign to my items , vm showed only the dreaded searchbox) was that the category list turns into a searchbox if you have more than 200 categories so i changed it to 1200 (yes i will have that much), maybe it is the same case for you, if you have a new installation, witha few items it works, and if you have already imported all your items (many many) then the listbox turns to searchbox?, i dont have that problem of the related items (for now) but here is what i did to fix the category assignment issue (very similar) maybe can send you to the right direction, or change that box tom a dropdown or whatever
## Edit file: /...ts/com_virtuemart/html/product.product_form.php
## category listbox was not showing in backend category assignment
## instead replaced with searchbox
##changed:
##line 256
## if( $number_of_categories > 200 ) {
## to
## if( $number_of_categories > 1200 ) {
## now it shows my listbox with everything inside it, no searchboxes
so maybe try searching for something similar in your case, maybe a hardcoded limit.