The primary diff is that subgroups are completely separate and new audio source while a VCA is simply a remote control for a group of faders.
Example: 4 channels of drums, kick, snare and 2 overheads
Assign them to a subgroup and you now have 5 audio channels. Typically you route the subgroup to LR output and unassigned the 4 individual drums. If not, any changes using the subgroup will prob be unnoticed since the orig audio ch is still in the output. Since the subgroup is a separate audio source, you can also assign effects etc to this audio. Use a subgroup when you need to add effects to a group of channels as a single sound eg. Compress the entire kit
Option 2 - assign the 4 drum ch to a VCA, you still only have 4 ch of audio. The VCA contains no audio. The 4 drum ch must still be routed to LR output. The VCA is simply a single fader remote control for the 4 drum ch. same as using 4 fingers to adjust the drums at the same time. Use a VCA when you only want to remote control a group of faders.
Hope this helps