differential vco
1. A differential VCO doubles the signal swing thereby increasing the carrier power. So Phase noise improves. Moreover the substrate noise gets canceled within the VCO., does not introduce current spikes on the supply line, low harmonics and lower supply pushing.
2. The load that the VCO sees must be symmetric as well., or else the imbalance will show up as phase noise (and in other parameters as well). Your idea works., but such a stage will have different input impedances for VCO frequency at the two inputs.
3. You lose on phase noise. It is better to use atleast one or two CML dividers, so that you don't introduce high frequency substrate noise. In most cases this solution is justified by power consumption. It also becomes easier to convert to CMOS after division.
4. Depends on your frequency output.