Simple set up:

Vader (w/ Advanced Target Computer) has target lock on Rey in the Falcon; Both Rey and Biggs are in Vader's arc and in range. Does Vader HAVE to attack Biggs or can he attack Rey due to the target lock?

I have been prancing on Squad Benchmark and in this scenario Vader is allowed to attack his choice, however I feel he must attack Biggs.

His primary weapon can target Biggs, so yes he must attack Biggs.

Is there anything saying he can't attack Biggs with his primary weapon? No? Then he has to attack Biggs with the Primary weapon if that is what he was going to use to attack a target near enough to Biggs. Just because that attack may have gained some added benefit for having a TL on the target is completely irrelevant.

If you had Biggs behind a TIE/sf with the title and there as another ship in front of the TIE but within R1 of Biggs the TIE/sf would still have to choose to target Biggs with an attack out of the rear firing arc negating any use of the title in that situation.

Cool. Thank you both for confirming.

The only way having a Target Lock on somebody next to Biggs gets you out of attacking Biggs is if you're using a secondary weapon with an Attack (Target Lock) header.

1 hour ago, digitalbusker said:

The only way having a Target Lock on somebody next to Biggs gets you out of attacking Biggs is if you're using a secondary weapon with an Attack (Target Lock) header.

...as, once you have selected that weapon, Biggs is not a legal target. (No reason not to say why, for posterity).