stop advertising virtual address when pool members down

If i'm understanding correctly, you have two different IPs, one at each site ... if that's correct, and if all clients use hostnames to access the VIP, then the usual way to manage this would be DNS. If you don't have a global server load balancing (GSLB, like is provided for by e.g. F5 Big-P DNS Controllers), then you would simply change the resolution in your DNS system to the backup site's IP, and back to your primary after. Note that if the DNS records involved don't have a short TTL, you'll want to shorten the TTL ahead of time (e.g. to a minute or whatever), so clients don't wait long to get the new IPs at failover/failback. You can set the TTL back after - but probably don't want to, since if you really did have to failover to the backup site, you'd want the resolution to change quickly anyway, to meet whatever SLAs you have.

If the IPs are the same, like described in the above reply, or if they're different but clients use the IPs directly to reach the VIPs, that won't work. May want to detail out your situation more fully if so.