The "typical" and "best practice" for J4 CSS override is to create a child-template, then create a new CSS file named "user.css".
When you do that, the file is found here:
/media/templates/site/(My-Template)/css/user.css
User is the only name that will work with J4. If you name the CSS file anything else, it won't work.
That structure in J4 is not related to the override suggested by Virtuemart.
So again. Where in J4 using VM4, would the "correct" path to override Virtuemart CSS be found?
Using what's been suggested here is not working.