VCCA must have 2.5V (+/- 5%) applied to it even if you're not using the PLLs. Whether you need a 2.5V power supply on board to supply that is a question worth asking. Assuming you have a higher voltage regulator you could potentially pot-down with two resistors to bias the pin at 2.5V. This relies on a stable rail that's not going to vary beyond 5%.
However, I wouldn't advocate doing this. For the sake of a very cheap linear regulator and ceramic cap, I'd add a dedicated power supply for peace of mind.
Cheers,
Alex
