VS with another VS as Pool

So to be clear, your external pool member is a VIP address?

I've heard people say that they've gotten this to work in some BIG-IP versions, but technically it isn't intended to work, and it has to do with the internal ARP between the VIPs.

To get from one VIP to another, your best bet is to VIP target, which is a simple iRule command:

when CLIENT_ACCEPTED {
    virtual VS-Internal
}

Also, depending on your BIG-IP version, you can also do this in an LTM CPM policy by configuring the policy to forward traffic to a virtual server.