Vengeance Landspeeders are Heavy Support. At least they were last edition.
I'm going to basically summarize what ntaw said, since he knows his stuff.
Detachment: A type of Force Org Chart.
Formation: A type of Detachment where most of the options are already made for you.
Battleforged Army: As many Detachments and/or Formations you can fit in your points without going outside of the Force Org Charts for each.
The Ravenwing Detachment can do the following:
Elite: RW Command Squad
FA: Nephilim Flier, Dark Talon Flier, Bike Squad, Attack Bike Squad, Landspeeder, Darkshroud
Heavy: Land Speeder Vengeance
None of the above, if taken as part of the Attack Squad, can be used in another Detachment. But as long as you run Sammael + 2 of those Fast Attacks, you can also use other Formations, such as the Silence Squadron (the 3 fliers), the Support Squadron (3 Speeders + Vengeance/Darkshroud) or Attack Squadrons (1 bike squad + optional attack bike + speeder).