Oscillator capacitors to VCC
If the onchip oscillator is an NFET (grounded source) with current-source pullup, there is some power supply rejection, and having the two PI network caps attached to Ground should produce a cleaner (less jitter) XTAL oscillator.

If the oscillator has an Inverter (CMOS inverter) gain approach, which has poor PSRR to GND and to Rail, then cap attachment likely does not matter.

Thus the answer is "it depends" on the oscillator circuit.