iRule to update vip address that DNS entry is resolving to.

The client DNS request should always go to your GTM, assuming the GTM is authoritative for your domain.

The subsequent data connection attempt, whether it be HTTP, HTTPS, FTP, etc. will be initiated towards the answer (cluster IP address) that your GTM hands back to the client.

Just for clarity, the GTM is your DNS server. GTM would answer client queries with whichever cluster is deemed healthy based on a monitor applied to the pool, and also taking into account the load balancing method you select.