Hi Adam -
You didn't mention if you are using the older type of SQL Impersonation (powered by "EXECUTE AS") or the relatively newer Kerberos impersonation/delegation. In both cases, there are extra steps you need to follow above and beyond what you've mentioned above.
My advice would be to take a look at Server Help - you'll find step by step guides on how to setup "old school" (EXECUTE AS) or "proper" impersonation.
Here are some links to get you started:
Newer: Configure Kerberos
We were able to resolve the issue by choosing the "server run as account" instead of "Impersonate via server run as account" and that did the trick,
Based on what you wrote, you're not really doing impersonation right now - You're simply having ALL users connect through a single Windows Account (Run As). vs. actually connecting to SQL Server using their OWN Windows Accounts. If that's cool with you, then you're done!
If you really want impersonation, you should KEEP the original setting you had in place, but then go into SQL Server and specifically grant the RUN AS Windows account permissions to EXECUTE AS for each and every one of the Windows Accounts your users will hit SQL Server with from Tableau. This is detailed in the link I sent you earlier
Thanks lot man for all the details,