No, nothing to do with cows...In Colombia, people use "vacas" as an abbreviated way to say "vacaciones." I haven't heard people from other countries use it in that way, though. Usually "vacas" just means cows. We (just Chili and I) are headed to Arizona for a week of fun in the sun with the grandparents. No cows included. At least none that I know of.
2 days ago