The EU VAT Assistant doesn’t include explicit support for multi-vendor platforms, such as Dokan. It should work with it, but it relies specifically on the WooCommerce logic to do its job. The best way to verify if the plugin can work for you would be to give it a try (it’s currently free, after all).
To answer your questions:
1. If buyer enters a valid ID#, it automatically gets checked at checkout before processing payment
Yes. The EU VAT Assistant checks EU VAT numbers automatically.
2. If it’s valid, do the taxes get removed?
Yes. The EU VAT Assistant applies a VAT exemption to the whole order when a valid EU VAT number is entered.
3. If it’s valid, does the invoice show the buyer’s VAT ID #?
The EU VAT Assistant appends customer’s VAT number to the billing address, which should then appear on invoices automatically.
4. If it’s valid, does the invoice show as “tax exempt”
No. The EU VAT Assistant doesn’t add VAT exemption information on invoices. You can use some custom code to tweak the invoices as you need (see https://www.ads-software.com/support/topic/change-label-vat-on-order-invoice/).
5. If it’s not a valid number, what shows up for the customer?
When the customer enters an invalid VAT number, the EU VAT Assistant shows an error message on the checkout page.
6. To prevent fraud, does the plugin only allow the VAT ID# to only be used for 1 customer account?
No. Any customer can use a VAT number. The EU VAT Assistant checks that the number matches the selected billing country, but it doesn’t check if multiple customers are using the same number. This kind of logic would have to be added with a customisation.
7. My site will sell subscription plans (recurring yearly). If the customer’s VAT ID# become invalid after some weeks/month, will i get a notification
The EU VAT Assistant it supports the official Subscriptions plugin and it checks the VAT number when the subscription is renewed. If the number is no longer valid, or it cannot be validated, the plugin doesn’t apply a VAT exemption to the renewal order and adds a note against the order. It doesn’t send an email, or a notification. This feature would have to be implemented as a customisation.
8. Does the plugin include advanced reports? If so what are they?
The EU VAT Assistant includes a couple of reports, showing the sales and VAT applicable to them. The easiest way to see what information they include is to run the plugin, then you will be able to find the reports at WooCommerce > Reports > Tax.