"Payment" should be the "Document Type".
But for VAT we only look at hte "Gen. Posting Types" sales, for the sales VAT, and purchase, for the purchase VAT.
For documents, like sales orders/invoices and for purchase orders and invoices the Gen. Posting Type is pre-defined.
In journals this can be selected. If it is left blank, then also the Postign Groups should be left blank.
Question:
Why do you want to register VAT when you post a "Payment" ?