Belgian and Dutch vAT number

If vAT is calculated on the invoice, I would use the Belgian number, otherwise the Dutch one. This makes no difference to the purchase amount, because you don't deduct the vAT anyway.

Has vAT been charged (because you yourself are VAT exempt)?

If vAT has been charged by the supplier, it doesn't really matter. Just book it as a BE company, for the sake of argument.

Thank you for your answer. Yes, vAT has been charged. I will just use their Belgian vAT number.