Since you are using FGT in policy mode I suppose you already have central NAT enabled, right?
When Central NAT is enabled, it is not necessary to add the VIP object into the firewall policy as the destination address. This is normal behaviour due to the fact that, in a Central NAT status, the DNAT is injected into the kernel since the object is created into the Policy & Objects -> DNAT & Virtual IPs.
Create the firewall policy and in the destination field, select the local IP configured into the VIP.
You can refer to the below KB for VIP configuration: