The tuning port capacitance for most of our VCO's can be found on the datasheet under 'Pin Descriptions' or more accurately what we sometimes will refer to as the 'Equivalent Pin Descriptions'. The pin description also typically includes any additional resistance or capacitance as well as the starting capacitance of the 'varactor' at 0V. This information can be used to simulate the modulation BW if the source impedance is known however this is still just an estimate. For the HMC587LC4B I conservatively estimate the Modulation BW at 17MHz. If this is a critical parameter for your system the best approach is to measure it. In synthesizer applications the PFD and loop filter will have an additional impact. I've attached an application note that may be helpful in making this measurement.