I just wanted to add my solution to this problem as I did not modify any of the root virtuemart code and it includes the custom fields.
It seems to work fine, should not be overwritten if you update virtumart and it is easy to revert to the original virtuemart later.
These are the steps
1 Create a folder rootOfYourSite/templates/yourTemplate/html/com_virtuemart/category
Files included in this folder will have higher preference ("overwrite") than the orginal virtuemart files
2 Copy default.php to the folder created in step 1
This is the original virtuemart file from components/com_virtuemart/views/category/tmlp/default.php
ecxept for lines 288 where I include addToCartCategory.php and line 291 which is commented out.
In line 252 I included the full description instead of short description of the product
3 Copy addToCartCategory.php to the folder created in step 1
This is the original virtuemart file from components/com/virtuemart/views/productdetails/tmpl/default_addtocart.php
except I did replace all $this->product with $product
Hope this is useful for someone
Would also like to thank the virtuemart team for making these modifications so elegant
Edit: Fixed typo in path and some clarification