Well, that is not that easy. Just a rough idea:

I assume you have 3 accounts concerning VAT administration, 1 for the VAT owed (sales), 1 for the VAT to receive (purchases), 1 for VAT settlement.

Create 2 additional tax codes, 1 for VAT artisans purchasing (tax excluded from price), 1 for VAT artisans sales with the regular tax rates and accounts.

Create an additional purchasing tax code VAT artisan write-off with a tax rate of 100% (or maybe 0%??) and a corresponding income account.

Create a write-off journal for this income account.

Now post a artisan purchasing invoice the regular way. For the reconciliation do a write-off in the same amount as the tax calculated using the corresponding write-off journal.

For filing the VAT return you calculate the corresponding tax base like (VAT artisan sales net amount - VAT artisan purchasing net amount) and the corresponding tax amount like (VAT artisan sales tax - VAT artisan purchasing tax).