How do I know which VCO will work best with the AD9510?

The VCO must be well-matched to the frequency and phase noise performance required by the system. The VCO datasheet should give important information such as the gain (Kvco), tuning range, and both close-in and wideband phase noise. Lower gain tends to produce loops with better phase noise performance but will constrain the frequency range. Wideband phase noise causes more jitter than close-in phase noise, because jitter is the result of the total integration of phase noise power over a broad bandwidth. However, system requirements may enforce low close-in phase noise, so this may be a consideration as well.