OK, meanwhile I discovered some things on my own:
Compared to http://www.howtoforge.com/perfect-se...ot-ispconfig-3
it seems to be required in Wheezy to manually enable libapache2-mod-fcgid to make ISP with 000-ispconfig.vhost work:
In addition to that (and completely independent from the problem above) also the following dovecot packages need to be installed on Wheezy: dovecot-mysql dovecot-sieve.
Nevertheless, what I'm still struggeling with: Why does the commenting in of the mod_php section in 000-ispconfig.vhost not work. Is there a access rights issue for apache or a different default symlink handling in Wheezy?