But then I'd also always assumed that Darth Vader had more "cosmic" things to do than robbing banks.
Perhaps the Imperial pension system has been slow on getting the checks out. Vader doesn't seem like the kind to automatically have 10% of his paycheck deposited in portfolio of balanced, low-fee index funds. The Emperor, on the other hand, is probably rather methodical about this, and has Admiral status in all of funds.
On a slightly more realistic (but still fantasy) note - this would make the perfect setup if there were two accomplices. Imagine the Darth Vader guy (properly dressed) has a accomplice in the crowd. When he goes to rob the cashier, the accomplice steps up and tries stop it and says "You're not really Vader", but Vader just says "I find your lack of faith disturbing." and the guy begins holding his neck, gasping for air, and collapsing on the ground (he's in on it, so he just runs out in pretend panic after Vader stops).
Vader then turns to the cashier (also an accomplice) does the Jedi hand wave and says "You will put the money in the bag". The cashier says back in a robotic voice "I will put the money in the bag" and does so. "You will not put any paint bombs or security devices in the bag". Then "I was never here". The cashier repeats both phrases. Afterwards, the cashier denies any memory of what happened or of Vader even being in the bank.
Sure, everyone will know it couldn't really be Vader, but you've introduced a lot of doubts way in the back of people's minds.
Still, from what I've read, robbing a typical bank robbery results in only $20,000 or so, and that's if everything goes really well for the robbers. It doesn't seem like a cost-effective way to make a long-term living, given the risks.