The datasheets aren't public, but there shouldn't be any difference in output quality. For Realtek, the letters at the end of the model name usually refer to the physical chip form factor. For instance the ALC1220 (no P) is a QFN56 package, while the ALC1220P is a QFN48. There is likely no practical difference between the two other than the number of pins. (Maybe a few GPIO changed or GND/power rearranged, or an extra feature removed; nothing that would affect performance of the chip itself).
There doesn't seem to be any consistent meaning to VB at Realtek. I've seen it refer to different digital signalling formats or newer processes, but never changes in core functionality. (The ALC5672 and ALC5672-VB, for instance, are identical as far as I can tell). Very unlikely to have any difference in output quality. If there's any change to the analog circuit, Realtek usually likes to start a new model number.
-CG means lead-free. Every ALC1220 should be -CG, whether or not it's marketed as such. A -T at the end, e.g. "CGT" means supplied on a tape and reel.
ALC1220X/S1220/S1220A are Asus-specific SKUs. No clue what they've changed. Probably something very minor, like having them pre-flashed at the factory so they don't have to reprogram the chips after assembly. The ALC1220X has marginally worse SNR specs in Asus marketing, but that could easily be due to something like locking it into amplified rather than line out mode, or them reporting the specs for the board as a whole rather than the chip itself.
Keep in mind that since the different SKUs are all so similar, a motherboard manufacturer using an "ALC1220P-VA3-CGT" may just call it an ALC1220 in their marketing materials.