Disable VS if only one pool member remains

Are you trying to do this for purposes of client connectivity (and disabling that) or to disable the VIP so it shows up down?

If just for connectivity, you could try an iRule and use the active_members command to check if there's more than 1 active member available. If not, you could just drop the request with the

drop

reject

You can also check out this question. I think it's the same as what you're asking.