A voltmeter is used to measure the potential difference between the two points of a circuit.It is connected across the two points between which the potential is to be measured.(i.e in parallel to the flow of current). The potential difference refers to two points. So to measure the potential difference between two points in a circuit, we connect the two terminals of the voltmeter to those points and hence voltmeter is connected in parallel. A volmeter has a very high resistance so that on connecting it in the circuit, there is negligible change in the circuit resistance (and hence does not draw appreciable amount of current from the circuit ). Its of no use to connect it in series as the electrical potential difference between two points at the same line is zero , which implies the readings will always be zero.
Hence a voltmeter is connected in parallel across the source battery in a transistor so that it gives the potential of the battery.