As the dewpint temperature is the temperature to which the air needs to be cooled to make the air saturated, the actual vapour pressure is the saturation vapour pressure at the dewpoint temperature.
cal_ActualVapourPressure_from_dewPoint(Tdew)
A vector for actual vapour pressure
dew point temperature(degrees Celsius).
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.