Don't know why I didn't think of it earlier, but you can see a map of the bus routes in Velizy (some of which go to major transit points - like the Gare de Massy or Antony) at this website: Plan du réseau Veolia Transport VELIZY

It's all in French, but click on "Votre trajet" on the left, and then on "Plan de réseau" - where there is a link to a pdf map of all the bus routes in Velizy.
Cheers,
Bev