Why is Vader's ship so bad? and falcon, for that matter.

Both the falcon and tie advanced are amazing ships. The falcon does insane damage and vader does great damage with his unavoidable hit. As stated several times in this thread they are glass cannons and they excel in that role in the fleet you shouldnt expect them to be tanks too. It sounds like you just need to learn how to use them effectively. Put them behind a tank(biggs has great synergy with vader), or stealth them, or buff the falcon up and use it to wipe the entire enrmy fleet in one pass.