Snubber to protect a VO2223A optical triac switching 0.5A @24 vac?
A full procedure for this task can be found in Fairchild Semiconductor AN-3008.  Going from that app note, as simply as I possibly can:


dV/dt(s) -- static rate-of-voltage-rise induced turn-on from transients is unlikely to be an issue due to the transient attenuation of the 24VAC transformer, unless long lines are present that can cross-couple transients into the triac circuit.  The use of a gate-to-MT1 (pins 5 and 6 on the VO2223A) resistor or capacitor should not be needed as the 210V/&micro;s dV/dt(s) rating of the VO2223A output triac is quite high; if this problem does occur in practice, a 100nF, X2 rated capacitor can be used gate-to-MT1 to suppress it.
dI/dt(c) -- the commutating current rate-of-fall is important for proper turn off of the triac, as it interacts with the dV/dt(c) of the triac. Fortunately, your load (0.5A at 60Hz) is not a stringent one, with a dI/dt(c) of .00018 A/&micro;s.
dV/dt(c) -- the commutating voltage rate-of-rise is the main factor a snubber is designed to control, as failing to control this leads to an inability to turn off the triac.  However, the correct snubber parameters depend on the inductance of the load, as that is important in determining the damping factor of the snubber, which should be kept around 0.3 to best suppress dV/dt(c) in a low voltage circuit.  Also keep in mind that the conduction time for the triac should be upwards of 5&tau; in order to allow the snubber capacitor to discharge.
Procedurally, based on the Snubber Physics section:


I'd measure the inductance of your coil -- any ol' LCR meter will do.
Pick a capacitor value based on whatever X2 rated capacitors you have available (I'm only using X2 here due to transient threats from long lines -- if that's not an issue, you can use a regular polyester film cap) -- anything between oh, 47nF and 470nF should be a workable starting point.
Compute the resistor value based on the damping factor equation given in AN-3008 page 6, and compare it to the minimum values in Table 1 on page 7 -- if you get less than 39 ohms, decrease the capacitor value to the next lower one available to you and try again.
TIAS -- measure the voltage and current waveforms across the triac, and make sure that you're within the VO2223A's dV/dt(c) rating.