@andreas-ph
I created a vendor account, listed a test product, and used the admin account to make a purchase using the WC Vendors test gateway.
The commissions were calculated correctly at 90%.
wp admin > woocommerce > commissions
One thing I noticed: if you vendors login using the icon link that buddyboss has in the main menu, they are not logged in properly. They will not be able to add images, and they will be logged out.
Have your vendors login via the woocommerce my-account page NOT the special buddyboss login. I really wish they would fix that!
For VAT- I have no idea how you will add this as you wish. The only tax settings we use are the standard woocommerce tax settings, which bases taxes form your shop location (admin) and settings that you enter in WooCommerce.
For the email troubles, please thoroughly go through our email debugging steps. The Postman SMTP plugin is especially helpful:
https://www.wcvendors.com/kb/debugging-mail-issues/