A set of four Cherenkov counters is currently operated on the roof of VATLY, a cosmic ray laboratory located in Ha Noi. Three of these are used to provide a trigger on extensive air showers to which the response of the fourth main counter can be studied in an unbiased way. The triple coincidence rate is of the order of 0.1 Hz. The showers selected are shown to be nearly vertical and to have energies in the range of a few hundred GeV with a core density on ground of the order of 1m-2. The response of the main counter to such showers is briefly discussed.
