VAT/Tax - don't include if valid VAT/Tax Number added

I am currently running Opencart version 1.5.4. I know there are much newer versions but just recently upgraded from 1.5.2 to add the VAT support - and wanted to reduce risk of breaking clients shop with upgrading to v3.

I have enabled the VAT (Tax) checker and this seems to work fine - tested with adding valid VAT number and a made up one.

How do I set it up so that if a valid VAT number is entered in checkout, it does NOT add the VAT to the total order cost? More specifically, if someone is from the EU and enters a valid VAT number, it will not add the tax.

(I used to have this setup using an extension but after upgrading PHP the extension no longer worked).

Thanks

Upgrade the php code of the extension

It was extension done by someone else who is no longer updating it since the functionality is apparently now in Opencart. It does validate the VAT number and give warning if its wrong, but I want it so that if valid VAT number added, then dont add the VAT? Can this be done? Or do I still need an extension?