I currently have both an Ash Tail Wolver Cap and Coat with Elemental defence Maximum. In the long run (when I can afford to have multiple sets of 5* equipment) would I be better off upgrading these to the Vog coat/cap or should I go for one of each of the other 2, or a specific one?
(To anyone wondering the armor is from back when you got 2 free alchemies a day and you could make a profit selling whatever you made).
Vog Cub is definitely the best choice here. Throughout the years it has been very clearly shown that regular defense (that is, not status defense) actually has very little affect in PvE. But even if it did have a noticeable effect, why would you be defending against elemental damage in a piercing or shadow area? That's a waste when you could instead go for much more elemental defense in an elemental area. Sure, there's the occasional area where two enemies are mashed together (a Lumber in a Wolver Den? What will you do?!), but overall you're better off maximizing one defense rather than several.
If you happen to care about Lockdown, in Tier 3 you're given 100 base defense and your armor defenses only matter if you surpass this-- even double Ele max Skolver or Snarbolax would have literally no effect on defense (IIRC), making them worthless unless mix-matched with another elemental armor. Just in case you happened to care for PvP.
too lazy; didn't read: use the defense UVs you have with their respective defensive armors. Choose Vog Cub.