The scanner setting you are looking for is called log_whole_attack which can be added like this to your openvas.conf:
log_whole_attack = yes
The location of the file itself depends on your installation method and can be found via openvas -s | grep config_file. If the file doesn’t exist it can be created at the related location.
Afterwards all info will be logged to either your syslog or the openvas.log (again depending on your configuration in the openvas_log.conf which should be found side by side to the openvas.conf previously mentioned).