Vyos do not send Advertisement Interval parameter in the Ipv6 router advertisement packet. Even if valid value is set to parameter Max router advertise interval.
'AdvIntervalOpt' by default is off for radvd.
In order to include advertisement interval info in the router advertisement packet this needs to be turned on
Put 'AdvIntervalOpt on' in the radvd.conf for specific interval to see this info getting advertised in the router advertisement packet.