vAT-deductible costs are the same as professional expenses

Interesting question, I think it should be allowed. For vAT, I think you will have to submit the actual costs, otherwise you will not be able to recover anything. But if you want to fall back on the flat rate for personal income tax, I don't see anything wrong with that. The flat rate is there, you have the choice to use it.Perhaps, but I'm not sure, you may be asked a question sooner or later. Because the vAT authorities will see that you are claiming X amount for vAT recovery and Y amount for personal income tax.

While it is allowed, I think, it is likely to raise questions :) Which is not a problem, of course.