I'm not sure this will work for everything. However, today I was performing an upgrade to a Joomla site with Virtuemart. When I completed the upgrade, I discovered that the virtuemart-ltr.css file has been overwritten in the component. I did not know that that file had been updated. Fortunately I was working on my development server!
So, I made a copy of the original virtuemart-ltr.css file and placed it in the CSS directory of my template, reloaded my page, and the Joomla system started using the virtuemart-ltr.css file that was in my templates directory, instead of the one in com_virtuermart/assets/css/.
Just a short quick tip when working with Joomla -- Check out and see if the component or module that you are working on supports this feature!