While VPN might allow you to get to your databases, that only indicates you can get to those machines via VPN - not necessarily Tableau Server.
Here's what I would do to try and investigate the issue:
- PING the Tableau Server by name: C:>ping myTableauServerName. Do you get a response?
- PING the Tableauy Server by IP address C:> ping <insert ip address here>. Do you get a response?
- See if you can surf the the Tableau Server in your browser
- Use a port scanning tool to make sure the port (probably 80) is accesible and open from your computer.
- Can your admin (inside the firewall, not over VPN) do the same things above?
This likely is not a Tableau Server issue, but a network, firewall, or DNS name resolution problem.
Hope this helps!