Hi,
It might be more appropriate to ask SourceFire this question, since it relates to how their product operates. However, based on what little info is available on their website and the attached diagram, it seems that the protected VM's share a common "protected port group" with the Inline Connection, and probably the same VLAN as well (unless the virtual appliance also does L3 routing).
Let us know how you get on with the SourceFire product. I'd be interested to know how it performs in the real world.