How do I set the commission I take from all my vendors, with a lower rate for one seller?

Set a global rate, then add an override for that vendor.

1. Go to Vendors → Commission.
2. Under Global default (admin commission %), enter 15 in Rate % and click Save.
3. Under Per-vendor overrides, choose the seller in Vendor, enter 10 in Rate % and click Add / Update.

To stop using the special rate, click Remove on that row.

When you run Sync from orders on Vendors → Vendor Earnings, each order is charged at the vendor's own rate, or at the global rate if the vendor has no override.