You can assign a particular vendor user as the sales rep. for a given LabOrders institution, by following these steps:
1) Go to menu Catalog > Institutional settings.
2) Find the line with the institution you want to assign the sales rep. to and click the Actions button on the right side of the row. Choose the Assign shipping policy and sales rep. option.
3) Choose the correct Sales rep. from the drop down menu and click Submit.