SDM does support VPN tunneling (though I have not used it for that purpose myself). Does your question imply that you have attempted SDM and it is not giving options to do VPN? Is this is the case, my first question would be whether the software feature set you are running on the 2621XM suports VPN.
The issue is not so much the version - 12.4(15)8T is good - but is a question of the feature set. If you are running something like the IP BASE feature set then it does not support VPN. You will need something like the Advanced Security or Advanced IP Services to get support for VPN. note that upgrading the feature set is normally something that you need to purchase. smartnet will cover upgrading versions but not feature sets.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...