First, a caveat: technically I've only done this work for PLD (as it's the only class I have at 50). I asked some friends and FC-mates to get naked with a sword to help shore up my findings with more numbers, which is always an interesting question. "Hi there, I'd like you to get naked. For Science."
Actually, it was pretty amusing to sit in the Ul'dah markets and take my clothes off, then put them back on one piece at a time, then take them off again, and so on. I had a few folks stop and stare at me for a bit, before moving on. I guess they assumed I was being hacked?
But doing some basic Health<->VIT ratio calculations showed that it wasn't precisely a X:Y ratio. It might've been linear, but there's a base value hidden. Taking off some gear, calculating my HP/VIT would give me a different answer than with all my gear. I needed to derive that base value and where it was coming fro, and that required knowledge of every variable.
My first order of business was to figure out how Bonus Vitality worked. Given that's a pretty big question for tanks in what exactly are you trading by taking less VIT and putting it into STR? If I wanted to answer one question and one question alone, that's a pretty big one. And I can get at least half the answer.
So I got naked, with no gear at all, and no PLD soul (so I was a level 50 GLA), and took samples. I'd mark my health, give a bonus VIT, and repeat. Here's the raw data:
Looks like 14.5 per point of VIT to me. And that held when I put my PLD soul back on. Nothing too difficult here.
Vitality from Gear
Next was to check how much health we get from gear VIT. Is it the same as Bonus VIT, or not? One might assume it would be, but I didn't want to make assumptions. I wanted to know for sure. So I bought some started gear, and figured out the early stuff, then I just slapped on end-game gear and interpolated from there:
So 14.5 HP per VIT again. Good! That makes things nice and easy. And confirms the Reddit thread.
Vitality from Race
This is where things got a little naked. I gathered data for Sea Wolf Roegadyn, Highlander Hyur, and Seeker of the Sun Mi'Qote:
From the data, if we assume that the racial VIT is 14.5 HP, since everything else has been, ideally then once we subtract it all, they should have the same base values. And indeed they do.
The rounding "error" you see is for the Roegadyn, 23 VIT means an extra 0.5 that would be missing from the display, bringing it to 1754 base HP. For the Mi'Qote, the bonus VIT adds a hidden 0.5 because it's odd, also bringing it to 1754 base HP.
So not only can we conclude that Racial VIT is likely also 14.5, but we can also conclude that GLA has a base of 1754 HP at 50. But what's the base VIT? Subtracting my racial starting amount puts my Roegadyn at 194 base VIT for GLA. No bonus, no racial, no gear. For my Mi'Qote friend, same deal, 241 VIT - 27 bonus VIT - 20 Racial VIT = 194 VIT.
But how does 194 VIT relate to 1754 HP? That's certainly not 14.5 HP per VIT. It's closer to 9.04. Well, for now let's move on. We'll come back to this question in a bit.
Vitality from Soul
Slap that PLD soul back on, time to see what it gives us! Going from GLA to PLD with no gear on, and with my bonus VIT, I gained 20 VIT, but my health jumped 460 points, from 2522 to 2982. That 20 VIT should account for a jump of 290 if we work under the assumption that 1 VIT = 14.5 HP. Where was the extra 170 HP coming from?
Same deal for a Hyur Highlander, with +30 Bonus VIT, went from 2508 to 2968. 460 HP. And Seeker of the Sun Mi'Qote, with +27 Bonus VIT, went from 2348 to 2808. 460 HP.
Clearly the soul was modifying our base HP value, and not just our VIT.
Base HP Value
Everything so far supports that 1 VIT is 14.5 HP, regardless of source, despite what some commenters were saying in the thread linked above. The only things we haven't been able to suss out are the base HP values with respect to base VIT. But maybe that doesn't matter. We know the base HP value is the same regardless of race, but it likely differs by class/job, thanks to the data we have from the PLD soul.
So perhaps we should just define 1 VIT as 14.5 HP, and come up with base values for our classes:
GLA -> 194 VIT * 14.5 = 2813 - x = 1754; x = 1059
PLD -> 214 VIT * 14.5 = 3103 - y = 2214; y = 889
Noting that the PLD soul always gives +20 VIT and +460 HP.
Which matches up with one of the commenters on the Reddit thread linked originally.
Putting it All Together
So for my Roegadyn PLD and all of his gear, his VIT is 531 and his health is 6810. If our calculations are correct, then I'd expect this to match up: 531 * 14.5 - 889 = 6810.5. Boom! Clearly FFXIV truncates, since my displayed health is 6810, but it seems like we're good to go.
The next question is does this hold for all classes? ie: Do all classes get 14.5 HP/VIT, and what are their base values?
But if you want to know if you should throw your bonus points into VIT or STR, you can now say how much HP precisely you're trading off. 435 health for a newly 50 PLD is pretty substantial (nearly a 10% boost), so don't sneeze at it.