The Collaborative International Dictionary of English v.0.48:

Vaquero \Va*que"ro\ (v[aum]*k[asl]"r[-o]), n. [Sp., cowherd, fr.
   vaca a cow, L. vacca. Cf. Vacher.]
   One who has charge of cattle, horses, etc.; a herdsman; a
   cowboy. [Southwestern U. S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

vaquero
    n 1: local names for a cowboy (`vaquero' is used especially in
         southwestern and central Texas and `buckaroo' is used
         especially in California) [syn: vaquero, buckaroo,
         buckeroo]