@kscholiadis You have to create a link between the Vendors and Transactions databases. I thought this was already done. If not here is a link on how to do that. About Connecting Multiple Database Collections | Help Center | Wix.com
You basically just need to create a reference field on the transactions database that points to the primary field of the Vendor’s database. Then your repeater will have access to the transactions database. Wix uses a SQL like relational database structure under the hood to manage datasets on their sites. So if you are familiar with creating foreign and primary keys this functions exactly the same.
If you are dealing with live data and need a way to refresh the database in real time you can just chain a .refresh() when updating the transaction total refresh - Velo API Reference - Wix.com