Hi there,
we are just in the process of migrating our stuff to a new C7000 + Virtual Connect (3.51) and Transparent Fortigate Firewall on ESXi.
What we try to do:
We got 2 uplinks (to the internet) from our DC.
UPL1 goes to VC1, UPL 2 goes to VC2. Booth assigned to vNet "upc_network", connection mode Failover. - this is the "outer" vNet
Also there is a second vNet "futureweb_network" - which should be behind the firewall server for our internal net
On Blade 1 we got a VMWARE ESXi 5 with an virtualized Fortigate Firewall VM in transparent mode.
That's how VC connects to this Server (I explain the deactivated Port 4 later):
UPC network connects to our first virtual ESXi switch:
Our Fortigate VM Firewall Port 1 connects to this switch and transparent filter/forward/scan all the traffic from this Port to Port 2 - which is connected to our vNet "futureweb_network"
All other Blades are connected to the vNet "futureweb_network"
I made a quick drawing of this Topo in MS Paint:
Everything works quite good - except when I activate PORT 4 for our Firwall Server. As soon as I activate the Port (within "futureweb_network" vNet the whole net goes crazy - loose all pings / everything is dead ... I guess it's causing a package storm. But I can't find WHERE we made the failure in this topo?!? Just want to bring every traffic from the outside through our Firewall into the inner net ...
I hope someone of you can help me on this! 
Thank you, bye from Austria
Andreas