VCO is not part of BPSK modulator. It is a part of PLL for non-synchronous demodulation of BPSK demodulator.

A simple phase-locked loop is designed to track a sinusoid. The VCO produces a sinusoid. When the loop is tracking properly, this VCO sinusoid and the input sinusoid have the same frequency. The multiplier produces a difference-frequency term and a sum-frequency term, but only the former passes through the lowpass filter. The output of the filter is an error signal, and it is amplified and then placed at the input to the VCO, completing the loop.