is it your job to maintain ac's at the apt complex *
if you have a fixed type metering device, measure the suction line temperature (insulate the sensor). compare that number to the saturated suction temperature (temperature your gauge needle reads). what is the difference between the two * this is superheat. an acceptable range could be anywhere from 7 to 20 degrees, depending upon other conditions.
if you have a txv type metering device, do the same thing with the liquid line - use the temperature number on the high side. this is subcooling. an acceptable range is typically 7 to 10 degrees.
you said you knew what the vapor line should be reading. should we assume from that there is a charging chart posted on the unit * if this is the case, it is correct to use it. this overrides any other method. charging charts will require two readings in order to intersect for a third value. if there is not a chart, tell us the metering device type, the superheat and the subcooling.
also measure the return air and supply air. the difference between these two is commonly referred to as delta-t and should be (ideally) around 18. as mentioned, make sure airflow is good and coils are clean before further testing.