After you've unlocked him to even be available to activate with Battle Points, of course. The 15,000 Credits price each to unlock Vader and other top-tier Heroes is not a tall order if you're only after a specific one - you could earn it in a solid couple of hours play, though that really adds up if you want everyone. Before the recent Hero price cuts, they were a whopping 40,000 Credits, which would take dozens of hours and yes, that is somewhat ridiculous.