cannot route traffic to virtual ip in another vlan

[johnie]

I would like to ask some questions for my issue.  My firmware version is V7.4.0 Build 2360.

1) I have FortiGate 600F and i made port23 as truck port and configured 2 VLANs on that port. These are. 

• VLAN 100 (192.168.100.251/24) interface name: vlan100
• VLAN 51 (192.168.51.251/24) interface name: vlan51

And I make virtual Ip address VIP on VLAN 100 interface, 

• name: DBVIP
• interface: vlan100
• external Ip address: 192.168.100.40
• map to 192.168.100.55 (this is my database server)
• port forwarding: 
• external service port: TCP 1521
• Map to Ip v4 port: TCP 1521

My question is that I want to access virtual Ip address 192.168.100.40:1521 from my client in VLAN 51 network.

My problem is that I cannot create firewall rule to access vlan50 to VIP address in vlan100.

How can I do that? 

For firewall policy,

incoming interface: vlan51

outgoing interface: vlan100

source: vlan51 network address

Dest: "VIP of DBVIP in vlan100 network"
service: TCP-1521

But i cannot found VIP of DBVIP destination option in firewall rules.

[johnie]

Any idea for my issue?