we just got the Cisco ASA 5540 which is replacing our Cisco VPN 3xxx Concentrators. as soon as i can i will try to MIB walk it and see if i can provide any help to you.
i do have our VPN sessions monitored on the 3xxx Concentrators using various OIDs to show IPSec LAN-to-LAN and OverNAT-T .etc plus Tunnel Count and Session Count.
but i don't know the ASA well enough yet but maybe real soon. maybe someone else will chime in with some info for you.
what i did with the Concentrator OIDs is build poller groups based on the tables and then put it together in the Node Details view. and you can create reports and publish these too.
I dont know if you have made any progress with this, but there are a couple of UDP pollers in the Content Exchange that will allow you to, at the least, graph the number of current (Active) sessions of your vpn sessions - I have ours set to so WebVPN, AnyConnect, IPSec and IPSec L2L.
Here are the OID's I am using on a Cisco ASA5520 -
SSL VPN (AnyConnect)
For anyone who's interested I've created a Universal Device Poller for the ASA to monitor VPN sessions using the information above. It's available in the content exchange at the link below. Enjoy.