Make sure that your pam_unix.so module comes after the pam_vas3.so module in your PAM stack, and make sure that the "use_first_pass" option is used.
This is a standard Redhat Enterprise Linux PAM configuration showing the default PAM stack:
auth required pam_env.so
auth sufficient pam_vas3.so create_homedir get_nonvas_pass try_first_pass
auth requisite pam_vas3.so echo_return
auth sufficient pam_unix.so nullok try_first_pass use_first_pass
auth requisite pam_succeed_if.so uid >= 500 quiet
auth required pam_deny.so