Pushing traffic through IPSEC tunnel with using Public VIP

I have a scenario whereby I need to push traffic through an IPSEC tunnel to a remote network.  This is not the standard type of setup as the remote subnet is a public IP subnet (80.x.x.x.x/28) and the remote party expects to see my traffic coming from some of my Public IPs.

A couple of these servers have 1:1 VIPs assigned for normal inbound traffic from the internet - when these talk to the remote network via the VPN the other side needs to see them as coming from the public VIP address associated with those servers.  Traffic from any other internal device needs to be seen as coming from the normal NAT address of the outside interface.

This is a bit of an odd setup which I haven't encountered before.  It appears that the ASA which I'm replacing was able to handle this just fine but I'm not sure how to approach it on the Fortigate.  Could anybody advise me on how I can get the result I'm looking for?

Thanks.