You can not use self signed certificate on the ASA for VPN Client authentication. Each VPN Client user requires his/her own certificate.
You can however use the ASA certificate server feature to issue each user with his/her own VPN Client certificate.
Here is the ASA local certificate server configuration guide for your reference:
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/cert_cfg.html