All that is known for certain is Juan de Vaca was present at the 1540 muster as an unassigned horseman. There are two possible choices for the expeditionary in the Pasajero record - one who went to Santa Marta in 1536 (as several expeditionaries did) and one who went to New Spain in 1538. More weight might be given to the Santa Marta individual because of an association with expeditionary Gerónimo Ramos. Ramos is listed right after Vaca on the muster and Ramos was in Cartagena. Perhaps they knew each other in Colombia. Both Pasajero individuals were natives of Mayorga, Valladolid province. Juan most likely served in Viceroy Mendoza's personal guard.
?AGI, Pasajeros, L.2, E.2499 (same as E.11878) [Juan Vaca, son of Villagómez and doña María Osorio, vecinos de Miorga [sic] to Santa Marta, April 20, 1536]
?AGI, Pasajeros, L.2, E.5613 = Boyd-Bowman, Indice geobiográfico, II:11679 [son of Nuño de Vaca and doña Beatriz de Quiñones, vecino of Mayorga, to México 1538]
?AGI, Justicia, 259, pieza 2 [visita a don Antonio de Mendoza] [19 Sept 1539, fol. 53r lists Juan Buca [sic, Vaca?] de a pie in Mendoza’s personal guard]
1540, Feb: Muster in Flint & Flint, Documents, Document 12 [1 horse, native arms]
?Catálogo, Resumen e Índices [21 Feb 1561, Juan Vaca, odrero, vecino, otorga poder general a Juan Sánchez Palma, procurador de causas, vecino. Firmó. Testigos: Juan Serrano, Antonio de Sarabia y Alonso de Alcohola, vecinos]
?Méndez Venegas, Emigrantes a América, #1783 [Extremadura, married Isabel de Carrión; agent for Francisco de Terrazas to collect money from don Diego Cortés, indio, 1561]
Other info: ?AGI, Pasajeros, L.1, E.2931 = Boyd-Bowman, Indice geobiográfico, II:8657 [son of Cristóbal Ballesteros and Catalina Fernández, natives of San Lucar de Alpechin, husband of Juana Polbillo, to Indies in 1526]