Hi,
You should do this modification via Virtuemart Backend. It is all about "Settings", no need to touch any core files. First you should add the "Euro" as the currency and then select that currency for products. If you have already created your products and selected USD, then you need to open each product one by one and change the currency from the currency dropdown.
Read the official Currency document for more info:
docs.virtuemart.net/manual/configuration-menu/currencies.html
Regards.