Quickbooks Desktop only supports the assignment of one Rep to one Customer. In contrast, multiple Reps can be assigned to that Account in Vinosmith.
 
When creating a brand new Account, Vinosmith will honor the Sales Rep that QB indicates. If this is changed in QB after the fact, you must initiate a Customer fetch so Vinosmith can reference the updated data.  This does not change the account assignment in Vinosmith.

You can use the "Re-Calculate Sales Rep Account Assignments" to update the rep assignment. However, this is an additive action, not a replacement.

For example, Account Acme has Alice and Bob assigned to it, and QB has Charles as the Sales Rep. Clicking the button would add Charles (if not already assigned), but it would not remove Alice and Bob. This is intentional. If there were any updates to the Vinosmith Account to allow additional Reps, then a replacement re-calculation would remove those updates. Thus, this operation is additive to preserve any Vinosmith modifications.

If you want to replace the rep assignment, you must make the changes in QB and Vinosmith.
 
Note:  Sales Rep Account assignments will not be pushed back to Quickbooks.