Hi Ben,
After some testing here, it would appear there is an issue with syncing Vendor notes for QuickBooks Online accounts. Although the 'Notes' field is contained on the Entity table as you observed, the notes added from QBO do not actually sync here, or anywhere within Method for Vendors at the moment (only Customers)
Ive brought this one up with the team, and Ill report back once we have an answer for you Ben.
Regards,
Ben