My current opinion of how to interpret Vader comes from the most recent ruling for Sense ... that you measure range to all viable targets, THEN choose whether or not to spend a force to look at a dial farther than Range 0-1. In Vader's case, you have to know what ships are available targets, before deciding to spend the force and activate the ability (maybe stripping a single token from a heavily-defended ship isn't worth losing that force token for a possible dice modification on defense).
EDITED ADDENDUM: Here's how I interpret how Vader works:
- In order to add Vader's ability into the Ability Queue, you have to make sure his triggers are valid at the time. The Requirement for Vader, as defined in the recent online ruling, is that there must be "1 ship in your firing arc at range 0-2...", at the Timing of "At the start of the Engagement Phase." This means that if there is no legal ship in the range, you cannot add Vader to the queue (even if an enemy ship later moves into range).
- When it comes time to Resolve Vader's ability, you defer to the Sense ruling, and check ranges to viable targets (your own included), then decide whether or not to choose a ship and spend the force. If you do spend force, the chosen ship suffers the effects... if you don't, they don't.
The only hitch I have in this, is whether Vader's ability, once added to the queue, is mandatory or optional. Once someone adds an ability to the queue, if it can be resolved, must it be resolved? Or can the controlling player choose to abandon an ability once it's in the queue?