Is 2" (5 cm) of workbench top overhang enough to accomodate most clamp-on vises and work lamps?
That should generally be enough. If you happen to run into a vise or lamp that wants a little more, you could edge-laminate an additional buffer strip onto the bend in that spot.
I would go further in the event that you want to clamp deep throat clamps or woodworkers clamps. Hand screw clamps can be 6" or more. Keep that in mind for the future. What about a single side with a deeper inset for specialty clamps?