Hi,
It is not "impossible", it is a doable thing.
There are 2 possible ways to achieve it;
1- User-friendly way;
Putting a "checkbox" to each product in the Virtuemart Backend, something like "Show Custom TAB1" and "Show Custom TAB2" would do the trick . But to do it, there should be a field in the database (extra field) and also a modification in some Virtuemart core files to show those checkboxes are required. Modifying the Virtuemart Core files or adding an extra fields to database are not good, because after updating the Virtuemart to any newer version, the modification will be gone. So this is not a convenient way.
2- Little Complex but more convenient way
Right below the "Custom TABs" fields in the plugin, there would be another parameter fields, something like: "Product ID's" (or Category ID's), you would list the product ID's or Category ID's in that field in a form of; 11,14,27,48.. So only these products (or all products of those categories) will show the custom field TABs.
The second method is a better and ideal way. I can write down this request for the upcoming version. But can't give you any estimate about it though. Or you can hire one of us through;
www.flexiblewebdesign.com/premium-support to have us to implement this custom modification to your website for you.
Regards,
Support & Development