*Posting on behalf of Kitru*

The only time in which you should use your allocated ability points for VIT is if you are absolutely trying to max your hp (e.g. you're undergeared and need to survive big hits). The reason for it is simple: conversion rates.

Allocated ability points are only time that VIT and STR can be traded on a 1:1 basis. On accessories, you can convert STR to VIT on a 1:1.1-1.2 basis (i90 accs are 15 VIT to 13 STR; i110 accs are 21 VIT to 18 STR). As such, if you've got full VIT allocation and then wear STR gear (30 VIT + 5 * 18 STR = 30 VIT, 90 STR), you're getting less total STR than you would get by going with full STR allocation and just getting 30 points of VIT from your accs (30 STR + 3 * 18 STR + 2 * 21 VIT = 42 VIT, 84 STR; that's trading 6 STR for 12 VIT and only because it's exceeding the 30; if you go with just 1 VIT acc, you get 30 STR + 4 * 18 + 21 VIT = 21 VIT, 102 STR; that's trading 9 VIT for 12 STR; either way, you're getting a better than 1:1 conversion rate by going with full STR allocation and using accessories as your conversion).

In short, you should always go full STR allocation unless you have less than the required minimum hp with full VIT accs (for SCoB, that's ~7200 for PLD and ~9000 for WAR w/ Defiance). Even if you're just barely short of it, I would still go full STR since 110 gear is incredibly easy to get ahold of and you're going to break that barrier soon enough anyways.

This also is a decent segue into discussion of the Topaz accs, which are BiS when pentamelded, even if you're trying to max STR, because of the generally accepted minimum hp requirements:

As a Sea Wolf Roe, with full STR allocation and full STR accs at i110 (no food), I've got 7748 hp (in Defiance) and 544 STR. I would need to devote 2 slots to VIT gear, which is going to drop STR by 36 down to 508, in order to break 9k with food and party buff.

If you go with pentamelded Topaz accs, on the other hand, you will get 15 VIT and 13 STR on every single accessory, which gives 519 STR and 9108 hp in Defiance (without food or party buff). By using pentamelded accs, you're getting 9 more STR and ~500 more hp than you would with pure i110 accs. The only time you want to use i110 accs is if you don't care about one of the two stats, which is either when you're not really tanking (e.g. t7 add tank) or you need to absolutely maximize your hp in order to avoid dying (MT Savage SCoB).