Change Tax Type

What I have already plan to do is to rename the vat field to a more generic tax.
But, doing so a system that permits labels customization would be necessary.
The invoice document by itself is already in a theme function.

>If there was some way to add a tax or change the current tax, that would be great.
You could use cck to add additional fields, of course.

I have implemented a system that permits labels customization now.
It will available in the next release.

I'll rename the current field from VAT to Tax and then every user will be able to customize it.