I noticed the same problem a few months ago and I contacted VB.
Basically, I live in a country that doesn't have a treaty with the US, so I pay 30% withholding on all US based sales (this is on SS, FT etc). On VB, their policy is ANY sales, no matter where in the world the client is, is deemed to be US income and subject to withholding at 30%.
I did some looking around on the IRS website and it seems to me that only royalty sales where the usage is WITHIN the US should be deemed "US Source income", for sales originating OUTSIDE the US, the income should be "Foreign Source" and not subject to withholding. This is the system Fotolia and Shutterstock uses (and some others).
I pointed this out to VB (and mentioned that if it is good enough for SS and Fotolia, it should be good enough for them) and was told they would speak to their finance department about it, they were very polite, but in the end, nothing changed.
So, for me, I have to pay 30% to the US government for sales on VB, no matter where in the world the client purchases it. This I think is wrong, but they will not change their policy.