The most important system performance criteria in selecting capacitors for XY applications are voltage, capacitance and temperature. The voltage rating determines the maximum AC mains voltage the capacitor can withstand and determines if a capacitor will be given an X1/Y1 rating or an X1/Y2 rating. With this in mind, the VY series is rated as follows: The VY1 series has an X1 rating with a voltage capability of 760 VAC @ 50 Hz and a Y1rating with a voltage rating of 500 VAC @ 50 Hz. The VY2 series has an X1 rating with a voltage capability of 440 VAC @ 50 Hz and a Y2 rating with a voltage rating of of 300 VAC @ 50 Hz. The capacitance value is generally selected such that noise suppression is maximized while not exceeding the maximum allowable leakage current as determined by the design. In regard to temperature, all of the Vishay VY series capacitors are rated from -40oC to 125oC. Please refer to links shown here for VY series datasheets and  general information for further clarification.