A couple of newbie tank questions

[Mertrodome]

If I'm in DF I'll usually do my first big pull in my full tanking stuff to get a feel for how quickly the DPS will kill things and how well the healer can keep up with the damage - that'll usually give me enough of a feel for whether I can switch into my DPS accessories or not.

For places where you might take gigantic pulls (either deliberately or because someone else facepulled - I'm looking at you, Aurum Vale scorpion things!) then you might want to stick with the Vit stuff there for safety.

I have two gearsets saved for each set up so I can easily switch mid-dungeon between different pulls - don't forget there's no cooldown penalty for doing so, so you can change whenever your not in combat.

With AoE stuff I'm not 100% sure as I only have a PLD at 50 at the moment myself. It seems to be later on for the melee guys whereas BLM is from very early on indeed. I'm sure someone better informed can come along and give you real advice on that part of your question

[Jican]

While there isn't a guide per say, you can certainly either ask, or look on the web for job abilities. You shouldn't really be truly speed running the lower level dungeons because you don't have the array of abilities and gear to handle it, and even if you did, it's synced down and you would only be able to hold out for a short amount of time before MP runs out. The exceptions are parties that have Blm's in them, since they are early AoE power houses. I will break down some of the AoEs of the jobs for you.

Monk- 1 AoE, very weak, used situationally for silence; 1 Cone, average, small cone and used in a rotation.
Dragoon- 2 AoEs, 1 is heavy on TP, the other has a semi-long cooldown. Average damage for tp move, heavy damage for long cooldown. 1 cone based, average damage tp move
Bard- 1 persistent move, lights the ground on fire, weak DoT; 1 AoE on TARGET TP move, average damage and debuff; 1 cone based, average damage, heavy TP cost, could proc a free AoE on Target move, or vice versa, I forget exactly.
Summoner- pet's have AoE moves, but each one is on a cooldown, and unless someone's using Titan-egi (and they shouldn't be in a party) they're not strong enough to pull hate. Miasma 2 is an AoE centered around the Smn, so they have to be right in the center for it to touch anyone. Shadow flare is a ground persistent AoE, like bard's ground fire, and it also gives a debuff of 20% slow for all mobs inside. Smn's really shine with Bane, which will take the DoT's they cast and throw them on 3 other mobs with the same timer the original mob has.
I save the best for last, Black Mage. They get fire 2 early, 20ish or so I think, it's been a while, and each astral fire they have gives a boost to damage, 20-80%. Our bread and butter is a massive hitting Flare at lvl 50, and unless the Blm know what they're doing, can easily take hate from a Pld.

I'm sorry I don't have the levels these come out at, I'm at work currently, but I hope this helps you get a general idea. Best of luck!

[Nyalia]

AoE: WHM get theirs at 45. BRDs get one very early, but it's terrible, and they don't get their better one until 50, I think. BLM can do it from super early on (earlier than 20 since I can use Fire 2 when synced in Tam Tara), and they really shine at endgame when they get Flare. SCH and SMN get Bane at 30 which is a limited AoE (4 targets, basically).

Really though, pre-50, AoEs aren't a good idea for any class in 90% of situations. They just don't do enough damage, even for the classes that have them. If a DPS is using AoE attacks against less than four enemies in non-endgame content, then they are not being as effective as they could be, and they should switch to single target stuff. When I see that happening, I usually say something (politely!) about it, especially when I see low level BRDs using their early-game AoE against a single enemy...

Regarding Str vs Vit, do you ever die in dungeons while the healer is still alive (doesn't count if the healer gets himself killed)? Do you regularly get below half health in dungeons? If either of those are true, run Vit accessories. If neither are true, go ahead and run Str accessories. If you find yourself in content where you are undergeared or clearly having trouble with your health, Vit is a necessity. Otherwise, Str helps both hold aggro and do more DPS.