Conflict with Yoast SEO plugin

The Yoast plugin and ours have different scopes, and they should keep working fine, without interfering with each other (please note that we don’t test plugins not developed by us). In fact, as we wrote in reply to another user, we use the Yoast SEO plugin on our site, together with the EU VAT Assistant, and everything works fine.

It’s possible that the issue lies elsewhere. Perhaps there is a JavaScript erro on the checkout page, and that prevents the VAT field from being displayed. I would also suggest to contact the Yoast team to see if they can spot any issue from their side, as it might be possible that any error could be due to a misconfiguration.