The vchkpw syslog facility is hard-coded, but changing it is very easy even if you're not a programmer. Simply open "vchkpw.c" in your favorite text editor and search for "LOG_MAIL". Replace that constant with another facility name as listed in "/usr/include/sys/syslog.h", for example "LOG_LOCAL1". The constant name appears only once in the file.
Re-compile using "make && make install-strip".
You do NOT have to re/start qmail for the change to take effect, because vchkpw is started once for every POP3 login.