In Twain's Adventures of Huckleberry Finn, "Blue jeans britches" is translated as "pantalones vaqueros azules"
In the US, I believe it was first miners who wore blue jeans most often; was it cowboy attire in Spanish-speaking countries?
It seems there is a chain of associations at work here. First, the cowboy tradition of the American West was actually derived from the tradition of the Mexican vaqueros, in turn brought from Spain. These vaqueros and their counterparts the American cowboys wore woolen pants with tight waists. Wool was later replaced by canvas, and rivets were added to the pockets to prevent tearing. Later still, canvas was replaced by denim, the material chosen by Strauss and Davids for their “blue jeans”. During all this time and through these changes, evidently, cowboys and their pants were closely linked in the popular mind, so that the pants were called vaqueros in Spanish. The popularization of Westerns cemented this link.
I have no idea what britches meant in Twain's time or in that particular book, but I don't think Twain was thinking of cowboy attire, and therefore pantalones vaqueros would not be a good translation, since the expression evokes the image of (modern) blue jeans or the kind of thing a cowboy in a Western would wear.