Sorry, there’s no such option for commissions at the moment, but this feature is on the roadmap. Setting a fixed commission would also require restricting the minimum allowed price.
If customers pay in person (not via your website) then it would be hard to charge vendors, unless you ask them to send the fee amount manually (e.g. every month). If you enable Bookings + Marketplace along with some “offline” payment gateway (like bank transfer or checks, you can also rename it to “Pay in person” or something similar) then there will be no payments made via your website, but you’ll be able to check the booking amounts.