Hello Eric
What kind of error message are you getting? Can you also inform us about the VRF configuration you have?
In general, if you want to add a TACACS server from a particular VRF, you can do the following:
(config)# tacacs server TACACS-SERVER
(config-server-tacacs)# address ipv4 10.0.0.10 vrf MGMT
(config-server-tacacs)# key <shared-secret>
(config-server-tacacs)# exit
Here you can see that the particular VRF on which the server is can be indicated. Here it is indicated as MGMT.
If you need any more information, please feel free to let us know!
I hope this has been helpful!
Laz