From a Civil War story by Ambrose Bierce:
vedette
n.
5Fr < It vedetta, altered (infl. by vedere, to see) < veletta, sentry box < Sp vela, vigil < velar, to watch < L vigilare: see VIGIL6 [Obs.] a mounted sentinel posted in advance of the outposts of an army

"a mounted sentinel posted in advance of the outposts of an army"

Doesn't 'vanguard' [Middle English vandgard, from avaunt garde, from Old French : avaunt, before (from Latin abante. See advance) + garde, guard (from garder, to guard. See guard).] have a similar meaning? Or is it employed during the movement of an army, while 'vendette' is used in the context of a 'stationary' army (outpost)?

Dear anchita: I think it is a difference in scale. A vanguard is an appreciable portion of the army advancing, to prevent successful ambush. The story I was reading was about a relatively small cavalry unit, and just three men were the "vedettes". apparently sent ahead to detect any enemy cavalry unit capable of attacking while the squadron ws in terrain where they could not maneuver. Unfortunately I got a "Page not found" and so was able to read only one page. Now satellites and drone borne TV cameras are the
vedettes. Bill

I have some interest in the Peninsular Wars. Wellington's army placed vedettes out in exposed areas, either between the two armies or out in the direction the enemy was thought to be likely to come from. I don't think there was any set size for a vedette. Depended on circumstances.

vedetta is , for example, even on a ship, crying "land" when he sees it.

Here in avan-guardia, guardia means a (small part of) an army.

Thanks for the clarification, folks!

Just to muddy the waters, vedete means "starlet" in Brazil.