@User Each VCO that is being used at a given time should have its own LDO that is not used by anything else that handles any signals or clocks, or otherwise has a varyng supply current. You could share a LDO between an active VCO and another one that is not in use.

if you are considering the VCO as a part of PLL, even if LDOs are powering VCO, noise could still be worse due to contribution from the other blocks. VCO impact the far off noise and the power supply noise sweeps in through the low frequency blocks on PLL. is it not ?

Yes, noise and spurs can get in through the supply rails for the reference path, feedback dividers, PFD/Charge Pump etc. as well as the VCO, so many of these will want separate LDOs in order to minimise spurs, but on the chips that I was involved with, the VCO was the block requiring the quietest supply rail.