hi,
first of all i'm using slackware64 13.37 current.

i have a connection to my workplace through vpn. i can connect with l2tp or pptp protocols. on windows 7 I was able to create a connection and connect from outside. was very easy tough.

on my linux system i installed networkmanager with openconnect, openvpn, pptp and vpnc plugins.
tried them all out but couldn't connect at all. i don't know why it's not working because there is no error description in the popup notification.

i also installed xl2tpd but it doesn't have a gui so i don't know how to configure it.

as much as i know there is a cisco secure acs v3.3 (tacacs+) behind the vpn.

The NetworkManager VPN plugins report errors to syslog, so check /var/log for the reasons they're failing. As far as NetworkManager-pptp is concerned, did you also remember to install ppp and pptp? I've never used L2TP so I can't help you there, sorry.

I have a very similar setup for accessing the network where I am employed. I found a couple of issues which I cured as follows:

Use kvpnc as a 'controlling client' (for want of better terminology)
Got my IT department to send me the cisco.pcf file and imported that directly into kvpnc
At that point it still did not work, and had to disable the firewall at my ISP to work from home, although working from the likes of a well know burger chain and any other number of internet hotspots gave no such problems.

it works now.

had to check mark "Use Point-to-Point encryption (MPPE)" & "Allow stateful encryption" under advanced vpn settings