Phase lock loop frequency of reference signal
However I am wondering what if I do not know the reference signal frequency, how can I approach the problem in that case ? How can I choose the frequency of VCO?


The VCO frequency should be equal to the desired output frequency, or cover the range of desired output frequencies. It isn't related to the reference frequency.

But to go any further, you do have to know your range of accepted reference frequencies. Because you'll need to choose a divide ratio to divide down the output frequency, and possibly the reference frequency, to obtain two equal frequencies for comparing to each other.