There is no documented or supported option in VyOS to tune internal conntrackd SYNC timers such as PurgeTimeout.
In VRRP-based stateful firewall scenarios, this means VRRP timing must be aligned with conntrackd behavior. Since conntrackd purges synchronized states after 60 seconds by default, the VRRP preempt-delay must be configured to ≥ 60 seconds to ensure that mastership is restored before the conntrack state is purged.

https://manpages.debian.org/testing/conntrackd/conntrackd.conf.5.en.html#SYNC

Sync {
	Mode FTFW {
		ResendQueueSize 131072
		PurgeTimeout 60
		ACKWindowSize 300
		DisableExternalCache no
	}