Once you head to the ITA search engine, you'll enter "PVR" to make Puerto Vallarta your destination or "MEX" to make Mexico City the destination. Then in both of the advanced routing code fields enter "aa."
Below those fields, check the box that reads "see calendar of lowest fares." Make your length of stay "3-10" and you'll wind up with a handful of available dates at these rates. They're mostly found in early to mid-March, so your vacation is coming up soon if you take advantage of these prices.