A *vara* is three feet, not three yards; that is, 1 vara = 1 (Spanish) yard. The Spanish foot (pie) was somewhat smaller than the standard English foot, but it was not standardized and varied somewhat from place to place and region to region. The RAE defines the vara measurement as: "Medida de longitud que se usaba en distintas regiones de España con valores diferentes, que oscilaban entre 768 y 912 mm." (http://lema.rae.es/drae/?val=vara). The most commonly cited modern equivalent is 836 mm (= 33 inches).
It's three foot, not yards, and it's an approximation. It really depends on where and when if you want to be exact. Kind of standard was the Burgos vara of 0.836m, but there were regional differences. Cuban vara around 1835 was given at 0.848m for example (Desiderio Herrera, Agrimensura aplicada al sistema de medidas de la isla de Cuba). Are you working on peninsular or colonial property? I guess for most colonial cases, you can only use approximation, and in that case I'd recommend to go for the Burgos vara, since it seems to be the point of reference for others. Hope that helps.