1 invoice (expenses) with different vAT percentages.

No, you can do it in one go. Just fill in the exclusive and inclusive amounts for the expenses and that's fine. It doesn't matter whether it's 6% or 21%, it's just the difference between excl. & incl.

And you can enter that in Dexxter, so there's no need to enter it multiple times.That's correct as far as the same cost categories are concerned. However, different vAT rates usually also mean different cost categories. That doesn't work in one go, so you have to upload the invoice several times. Hopefully, Dexxter is working on making this possible. .