DNS issue: Tor works great but VPNs do not - why?

[Matthew]

  I am on a University network.  I do not have control of my DNS settings.  
When I use OpenDNS, for example, I cannot get any connection.  This is 
understandable.

Tor works fine and, as we know, DNS resolution is done at the last exit node.

However, I cannot use a VPN.  I have tried Perfect Privacy and SwissVPN.  
My understanding is that VPNs also do the DNS resolution at the end of the 
tunnel.

Can anyone tell me why, if both Tor and VPNs do the DNS resolution 
remotely, that Tor works but VPNs do not on my network?

Thanks.
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