When the following command "set system syslog file <filename> facility <keyword> level <keyword>" is applied, then the files are stored in the /var/log/user directory. These files can be deleted using the command "delete log file <text>"

Updated the documentation:

https://docs.vyos.io/en/latest/configuration/system/syslog.html#delete-logs

vyos@R3# run sh conf comm |grep syslog
set system syslog file test facility auth level 'all'

vyos@R3# ls -ltr /var/log/user
total 4
-rw-r----- 1 root adm 561 May 30 08:50 test

vyos@R3:~$ delete log file test
vyos@R3:~$ ls -ltr /var/log/user/
total 0

The file will be recreated once new events are generated.