So /etc/sysconfig/spamassassin should be like this:
SPAMDOPTIONS="-i 127.0.0.1 -A 127.0.0.1 -v -d -p 783 -u vpopmail"
then all works fine:
spamd: Jul 16 21:57:28.013 [3933] info: spamd: connection from 127.0.0.1 [127.0.0.1]:45154 to port 783, fd 4
spamd: Jul 16 21:57:28.030 [3933] info: spamd: processing message <000601d53c10$b1f1e120$15d5a360$@wp.pl> for info@xxx.eu:89
spamd: Jul 16 21:57:28.241 [3933] info: spamd: clean message (1.2/5.0) for info@xxx.eu:89 in 0.2 seconds, 3709 bytes.