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]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "vaquero":
   broncobuster, buckaroo, caballero, cattleman, cavalier, cavalryman,
   chevalier, circus rider, cowboy, cowgirl, cowhand, cowherd, cowman,
   cowpuncher, drover, equestrian, equestrienne, gaucho, goatherd,
   gooseboy, gooseherd, herder, herdsman, hogherd, horse soldier,
   horse wrangler, horseback rider, horsebacker, horseherd, horseman,
   horsewoman, jockey, knight, mounted policeman, pigman, postboy,
   postilion, puncher, rider, roughrider, sheepherder, sheepman,
   shepherd, shepherdess, steeplechaser, swanherd, swineherd,
   trick rider, waddy, wrangler