Assuming you are using the ‘classic’ L2TP/IPSec VPN Client then unfortunately there is no way to assign a policy based on a user. The closest you’ll be able to get is manually assign a Group Policy to a client once they are connected. Each time they reconnect that policy will be applied to the client.
If you’re game enough to try the MX16 code then you can use the AnyConnect VPN Client. Using the AnyConnect VPN Client you can perform RADIUS authentication and return a Filter-ID attribute which can be used to apply a Group Policy to the client.