Although this topic has not been visited recently, I came up by searchangin at this page so I thought someone else might benefit.
1. I added in custom css file optional for my template:
.product-field,product-field-type-R{width:25%!important;
float:left!important;} - this keeps it untouched in cas of an upgrade.
Alternatively step one might be added directly in vmsite-ltr.css, but this will be changed when upgrade is made.
2. Find file ........... /components/com_virtuemart/views/productdetails/tmpl/default_relatedproducts.php
add at the end
<div style="clear: both;"></div>
This solved it in my case.