The brake booster and the vacuum pump aren't the same thing. But the car needs them to work together for normal braking. That "brake booster vacuum pump" you mentioned, in some cars, to save space and make things more convenient, they combine these two into one part. Since the BMW repair shop said there's an oil leak at the seals of the brake booster pump, it's likely that the seals in this combined part or those related small components are worn out. I suggest you get it fixed as soon as possible. Otherwise, it might mess up your brakes.