2005 Cadillac STS 4.8 North-star engine P0174 code came on..

P0174 is bank 2 lean. I would not suspect a vacuum leak, if there is not an accompanying P0171 -- as a vacuum leak would affect both banks, not just bank 2. I don't know how low fuel level could affect the bank 2 oxygen sensor reading alone. I think you'll need to go further into testing, first into the concern you are having, and then into the set trouble codes which could give clues.