OK - we got it working and everything now works as expected. The problem turned out to be something to do with the installation. During the installation, your customized version of the ps_product.php file did not overwrite the one in the administrator/components/com_virtuemart/classes folder. After manually copying your version of this file to this folder, everything worked.

Simon,

I just tried that and it almost solves it.
Filling the VAT field with an german VAT, VAT is still calculated.

So almost, but not entirely

Are you sure the problem is not as above?

Did you have correct file permission settings set on installation of the checking code, so that the original files had been overwritten correctly?
You can check that manually. Simply open th installation ZIP archive and overwrite the files manually.

Yep, sure.

I just uploaded the ps_product.php to the administrator/components/com_virtuemart/classes folder overwriting the original. Same result.
I tested it with VAT DE118646776, Germany and VAT NL806078236B01, Dutch.

2 more questions:

1. what was the address of the shopper with the respective VAT IDs?
2. did you try with VM debugging mode on? (if not, please test it and report on what condition the test fails)

During shopping and at the registration, the following statements came up:

Debugging: shopper's country is not known; defaulting to vendor-based tax
Debugging: calculating tax based on vendor address

Aparently the country doesn't get through. I deleted cookies on every try.

The address of both VAT's are good. Country, phone, mail. All correct.