how to set department value in vendor bill on tax line? I suppose it should be inherited from the item/expense line but in my case tax line dept stays just empty.

Does doing so make any sense?

According client's requirements all expenses must have department value assigned. And in this case tax amount goes to expense account.

Is the header department exposed? If so, is it populated?

its is populated on a line, and expense line has department field populated as expected. Question was how to get the same on a tax line