With VSX, a given VLAN on a given physical interface can only go to one VS. That is, you can't have six VSs all sharing one physical interface to a core transit VLAN. This is where you need a virtual switch. If your six VSs will all use different physical interfaces, or different VLANs on the same physical interfaces, you don't need a switch.
One thing the VSX documentation and introductions don't really cover: use bonds for all of your interfaces. Don't let VSX be aware of any physical interface which isn't in a bond. Stick the physical interface in a bond, then tell VSX to use the bond instead. This helps a LOT when you eventually want to replace the hardware. Bonds can have a single member, and non-LACP bonds with only one member don't need any special support on the switch side.
