This is a setup through multiple rounds question;

If Ship A acquires a Target Lock (all following requirements for eligibility of said action; range, etc.) on Ship B.

During the same round Target Lock has been acquired, Ship A never uses their Lock.

In the next round for "Perform Action" step, is Ship A eligible to stack a Focus or Evade on top of their Target Lock?

Is this a legal course of actions over multiple turns? Or is the Target Lock, until used, considered a sustained action over the following rounds, Disallowing Ship A to make any other Actions?

And if it is a sustained action, then here's a character specific follow-up;

With Darth Vader* being allowed to perform two actions in one turn, may he only be allowed to perform one action if he has a pre-existing Target Lock from a prior round?

Asking because there seems to be lack of explanation in rule books on this as far as I have looked.

Might someone clear this up?

So in short there is no such thing as a "sustained action" in X-Wing and you can stack as many things as you like as long as something gives it to you.

Performing a Target Lock action (or acquiring one another way, say from Fire Control System or any other number of upgrades) allows you to assign a Traget Lock token to any enemy ship out to range 3. The action is basically instantaneous in gameplay terms and is finished. Once the token is placed it remains on place until an effect removes it (being spent or acquiring a new lock on a different ship 99% of the time)

The same is the case with Focus and Evade tokens, the action is instantaneous and the result of the action is the appropriate token is assigned to the ship. Again the tokens are not removed until an effect calls for it but the action is not "ongoing" just because the token is present. However, the rules specify that during cleanup at the end of each round unspent Focus and Evade tokens are automatically removed from play. This is not the case for Target Locks so they remain.

One other thing to keep in mind, when an upgrade card says "Acquire a Target Lock" like say Fire Control System, the ship that upgrade is on may get the lock whether it has the TL symbol on its action bar or not. Additionally "Acquire a Target Lock" is not an action, otherwise it would say "Perform a free Target Lock Action" so you may do it even while stressed or any other condition that might prevent an action.

A Target Lock persists until used or removed. This is true even if the target of the lock at some point moves out of range.

Typically focus and evade tokens are removed at the end of a round (there are exceptions to this). Target locks do not expire at the end of a round.

As an action you can re-establish a target lock on a ship you already have locked if you choose to do so (and usually you would do that only if you gain some advantage, like the Inquisitor).

As an action you can move your target lock from ship B to ship C assuming ship C meets the restrictions (in range 3, and so on).

Darth Vader can target lock + one other action this round, and if he maintains his target lock, will get two unique actions during the following round while maintaining his target lock. He especially wants to do this if he has the Advanced Targeting Computer upgrade.