Don't have a sure-fire answer for you , but do have a couple of suggestions (as well as things I know won't work):
Things that won't work:
Trying to tie the VPN software to a user profile - since VPN software usually binds itself to your networkadapters (NIC and/or dial-up adapter), settings are applied to all users. I have never found a registry setting to tell the software to apply to only an individual user profile vs. all users.
Things you might try:
Since the VPN software binds to the network adapters, you might try multiple hardware profiles with a different VPN protocol disabled in each. However, I suspect that just having both clients installed (if in fact both will install successfully) will cause conflicts.
The onlysure-fire way I can think of to make this work (unless you can get the VPN vendors to offer a solution - fat chance) will be to create multiple-boot systems. The best way to do this (and easiest for you users to use) would be to use something like System Commander (from V Communications) and install multiple iterations of the same OS (with a single VPN client installed in each). Create a third partition that can be shared between the 2 installations , that can be used for data (working files)that needs to be accessed from both installations
Hope this helps.