Real,
I have Windows RT 8.1 and I don't see anything in the VPN connection properties indicating "full" support for IPSec groupname+password authentication, which is essentially what most companies using Cisco VPN seem to utilize. I could be wrong and I even hope I'm wrong, but I don't think so.
This link is frequently referenced:
http://technet.microsoft.com/en-us/library/jj900206.aspx
...but the only IPSec authentication supported is still through a certificate.
Feel free to post a screenshot of the appropriate Win RT 8.1 dialog to prove me wrong. Please prove me wrong.
Try creating the VPN connection by bringing up the control panel and put VPN in the search window and following the prompts; you may need to go back and edit after it fails in the first run through. I took a look at it, but don't have a VPN to test it on...