I have changed the Vendor Master entering Thailand Branch Details button (please see attachment).
If you enter the default Branch Code in the Vendor Master, you will have no problem posting invoice in FB60 or MIRO. The system will automatically use the default Branch Code NVAT.
For the beginning, please enter the Branch Code and Default Branch Code in the Vendor Master manually.
Meanwhile Carola (Data, SAP CC) is working on a mass change of all your vendors. After this you will not have to enter anymore Branch Code in Vendor Master manually.
Please start to use this solution now and let me know after the first invoices were posted. Thank you.