whenever a vApp node boots it will sync its time with the host it runs on and won't sync again until next boot (just like any ordinary server).
There are several ways to implement constant time sync in an environment and it is based on your preferences:
If you want to implement a node's constant sync against its VMware host, you should Install VMWare tools.
https://techdocs.broadcom.com/us/en/symantec-security-software/identity-security/identity-suite/14-4/virtual-appliance/configuring-virtual-appliance.html
Otherwise, If you wish to sync the nodes directly against an ntp server, you might use this guide to Configure Time and Network Time Protocol as described in the above link.