Setting up a POP server with the QMail/VPopmail combination wasn't too much hassle until the point I tried to authenticate.
Telnetting into the POP server and giving authenticating myself resulted in the following error:
Some people suggested to increase the maximum allowed memory usage in the POP3 init scripts (softlimit). But that didn't really work out. The following trick made the authentication work all of a sudden. Simply replace the path /home/vpopmail/bin/vchkpw to /usr/local/vpopmail/bin/vchkpw.
So this is the full POP3 script for the supervise daemon:
exec /usr/local/bin/softlimit -m 4000000 \
/usr/local/bin/tcpserver -v -R -H -l 0 -u 89 -g 89 0 110 \
/var/qmail/bin/qmail-popup pop.example.com \
/usr/local/vpopmail/bin/vchkpw \
/var/qmail/bin/qmail-pop3d Maildir 2>&1
If someone can explain why this works, please speak up 
I checked the MD5 hashes and the binaries are equal.