pdns_recursor stats logs are seen in journalctl but not in /var/log/*.
As journald doesn't save to disk this means all logs are lost on reboot.
pdns log facility is 3 (daemon) and priority (=syslog severity) is 6, this translates to daemon.info
Mon 2019-06-24 12:26:51.084882 CEST [...]
_BOOT_ID=xxx
_MACHINE_ID=xxx
_HOSTNAME=vyos
_TRANSPORT=stdout
PRIORITY=6
SYSLOG_FACILITY=3
SYSLOG_IDENTIFIER=pdns_recursor
MESSAGE=stats: 54106 questions, 4436 cache entries, 997 negative entries, 6% cache hits
_PID=2868
_UID=0
_GID=0
_COMM=pdns_recursor
_EXE=/usr/sbin/pdns_recursor
_CMDLINE=/usr/sbin/pdns_recursor --daemon=no --write-pid=no --disable-syslog --log-timestamp=no
_CAP_EFFECTIVE=404c1
_SYSTEMD_CGROUP=/system.slice/pdns-recursor.service
_SYSTEMD_UNIT=pdns-recursor.service
_SYSTEMD_SLICE=system.slicevyos default for /var/log/messages is *.notice;local7.debug
By setting set system syslog global facility all level info; set system syslog global facility protocols level debug, from short testing on my machine, it adds dhcpd and sudo logs which you'd want saved anyway, so imo the default should be set to *.info. This does increase disk writes so it may not be suitable for flash drives.