Comment out pam_selinux.so, the server does not start if missing.
Add pam_elogind.so for non-systemd installs.

See also:
Bug: https://bugs.gentoo.org/746227
Bug: https://bugs.gentoo.org/937283

--- a/unix/vncserver/tigervnc.pam
+++ b/unix/vncserver/tigervnc.pam
@@ -1,15 +1,13 @@
 #%PAM-1.0
 
-# THIS IS AN EXAMPLE CONFIGURATION
-# MODIFY AS NEEDED FOR YOUR DISTRIBUTION
-
 # pam_selinux.so close should be the first session rule
--session   required     pam_selinux.so close
+#-session   required     pam_selinux.so close
 session    required     pam_loginuid.so
--session   required     pam_selinux.so open
+#-session   required     pam_selinux.so open
 session    required     pam_namespace.so
 session    optional     pam_keyinit.so force revoke
 session    required     pam_limits.so
 -session   optional     pam_systemd.so
+-session   optional     pam_elogind.so
 session    required     pam_unix.so
 -session   optional     pam_reauthorize.so prepare