Security-wise it bears no meaning to use Any or specific interface, it just binds this object to be used on a specific interface to may be prevent someone from configuring VIP on the wrong interface and then wondering why it is not working (my personal idea of it).
I always set it to Any. Actually, in the case of multiple ISPs, when external IP used in VIP is your own, advertised via BGP to providers, you have to leave VIP as Any or failover/configuring the same VIP for both IPS connections would not work.
Thanks guys for responding...support also got back to me saying it should be fine. Im just bit worried, since we are converting to Fortinet from another vendor and there are lots of NAT rules we had to move over, so this is very critical to work right.