Yes to VC's being about specialisation. So what Civs are best at turning from one specialisation to another without losing much steam?
Which Civs are more likely to pull that off regardless of VC type? That to me is a good measure of versatility.
Yes, this is a good measure of versatility that only a few of civilizations can do. Also the ones that won't be so affected with bad situations. Some examples, Brazil can be very versatile with rainforest, but what he can do if he can't expand with the rainforest, can he survive with just the GP bonus? What can Rome do if he can't build many cities early on, can he catch up just with the bonus trading posts? What if Aztec don't get Luxury resorces? If Australia need to declare war to try expansion?
Can a Warmonger CIV still win if he doesn't get a war? Can a science CIV win if he couldn't invest in Science because he need to defend against barbarians or Warmonger CIVs?
This is Adapt and Overcome and deal with almost any situation. This is versatility. Of course all situations is impossible, but some CIVs can deal with ALMOST all of situations.
I'am kinda new in CIV6, but as I could play and read, I would say Germany and Japan, just because the fact that the most versatile tool in this game is the District. Germany can build one more district and Hansa can be good with any style you want, and Japan just ignore all of location situations while a lot of cheaper districts.
Edit: I still have faith in Brazil and Rome versatility just because of Street Carnaval and Baths, Amenities is the second most versatile tool, the Ecstatic city bonus is huge.
Also the other trait of a versatile CIV is the ability of taking opportunities, which can open another brand, but let stick with what we are discussing.