"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)?