Have you check our page dedicated to Authentication and access?
I'm sure it could help you out: http://onlinehelp.tableau.com/current/server/en-us/help.htm#authentication_access.htm
Let us know if you still require support
When using trusted authentication, the user account which is connecting to Tableau Server (via the portal) is still being authenticated, and is logging into the Tableau server.
All that is happening is that Tableau Server is not asking for and checking passwords for any requests that come from the trusted IPs (because Tableau trusts them)
So, as long as the user name that is being used via the portal is the same one that is setup on the SQL DB then there should be no reason why impersonation shouldn't work.
Please take a look at this article on SQL Impersonation requirements, as setting up the SQL part can be a bit of a pain
Therefore, it may worth taking a look at using Kerberos authentication (espicially if your Web Portal is running on IIS), and using Kerberos Delegation for SQL
Hope this helps