In general, we recommend selling vouchers tax-free as well as redeeming them tax-free, since redeeming the voucher must not change the VAT amount.
So that vouchers can be sold tax-free, a new VAT rate of 0% is automatically created in your shop when the plugin is installed, provided it doesn't already exist. You can find the tax rates under Settings → General settings → Shop settings → Taxes.
In the default configuration, Shopware lists each tax rate separately on the invoice. If the 0% tax rate should not be shown explicitly, the invoice template must be adjusted accordingly.