When forked to the background, hostapd is capable to use syslog for logging making it quite hard to troubleshoot issues on the Wifi phy and mac layers. However, I have found this feature to be disabled in all VyOS versions by this hostapd.conf setting: logger_syslog_level=0
Manually enabling this (for example set logger_syslog_level=4) and then restarting hostapd in forked mode still does not make it log to syslog.
I tested this on VyOS 1.1.8 and 1.2.x.
Related to: T452
Priority set to "LOW" due to capability of starting hostapd manually and debugging that way. Also, fixing this will involve patching hostapd which is very likely out of scope to the VyOS project. This might even justify a "Won't fix"verdict.