"I already work in a bank."
Translation:제가 이미 은행에 근무해요.
They both mean the exact same thing, just 벌써 carries a little feeling of something already happend a while ago. But they can be used interchangebly without worrying about the ectra feeling.
I learned from someone that 벌싸 is for something that only one party knows about where 이미 is for when both parties already know about whatever fact they are talking about.
It's difficult to explain effectively here, so I found a very useful stackexchange answer you can look at ^^