I've been running DTF a lot more lately, but none of these situations have come up, so I want to be prepared in case they do.

Someone shoots with an Ion Cannon, and rolls a crit and two blanks. The defender rolls 0 evades. But a nearby ally has DTF.

1. Can DTF trigger in this instance?

2. If DTF does trigger, who gets ionized?

Same question for Fletchette Cannon!

And Ion Pulse Missile, Fletchette Torps, Ion Torps, Assault Missiles, etc.

There is no reason to use DTF when someone is hit with a Ion turrets/Cannon, because even if you draw the crit off the defender, the defender will still suffer 1 damage and get 1 ion token.

Ion turrets/cannons trigger when a ship is hit, Draw Their Fire, doesn't change if a ship is hit or not, just how much damage it takes. Since the ion cancels all hits and does 1 damage and give 1 ion token, DTF would do nothing.

In theory you could actually suffer the crit on the ship with DTF on top of everything else, taking a extra damage for nothing.

All the rest of those things you list are based on a ship getting Hit, and DTF is not going to change what ship suffers the extra effects from those upgrades.

Flechette Torpedoes

Attack (target lock): Discard this card and spend your target lock to perform this attack.
After you perform this attack, the defender receives 1 stress token if its hull value is "4" or lower.

the stress goes thru regardless of damage, while DTF can take the single crit away the attack was still made and the defender still gets a stress, not the DTF ship.

Now,Darth Vader could work since it is afrer the attack.

The complete answer is a slightly uncertain but it is generally assumed you would NOT want to even if you can.

If the Ion Cannon "hits" then the target will take damage and gain an ion token. These things are independent of the actual rolls (beyond needing to hit) as the dice get cancelled anyway. The only way one could interpret things is that the target would still suffer the special effects of the Ion Cannon (damage and token) but then the ship with DtF would still suffer the [crit] result; this assumes the [crit] isn't cancelled along with all of the other Ion Cannon dice (but it should be) and most of us have hard time figuring why you'd want the enemy dealing more damage to you.

Now,Darth Vader could work since it is afrer the attack.

I've got no idea what this means. Passenger Vader's ability can not be moved from one ship to another with DtF because it is not caused by an attack but rather by the card's special ability.

1. Can DTF trigger in this instance?

2. If DTF does trigger, who gets ionized?

1) Yes it can.

2) The original target.

Using DTF against ion or flechette attacks offers no benefit. There is no consensus whether or not the use of DTF damages the DTF-ship in addition to everything that happens to the defender.

Against assault missiles, DTF shifts damage from one ship to another. That does not change the splash damage in any way.

This has been debated on this forum numerous times before. While the general concensus is that it would make no sense to do it if you could, there is still disagreement as to wether you even can trigger DtF or not. I disagree with dvor. Since Ion Cannon and other weapons like it cancel all dice, I say there is no critical result to suffer on the defenders behalf, thus not allowing DtF to resolve. But no matter if dvor or I am right, the defender still suffers one damage and gets one ion token.