My understanding on SSLVPN is as follows.
The main purpose of SSLVPN is to have access to the protected resource from "anywhere". By 'anywhere' I mean from any machines from Internet Kiosks, from your friend's home computer etc. So, the users will not have their certificates installed on these machines. The authentication will based only on the RADIUS authentication.
SSLVPN does not expect the clinet to authenticate. Only the server authenticates like in Internet transactions. But I also remember that the SSL protocol has an option to authenticate the client certificate.