if Oswald had opened the door just as Baker was coming up the front entrance steps on the right side of the handrail, where Baker runs into Pauline Sanders, then Sanders would have likely seen Oswald and so would Sarah Stanton aka prayerperson, have seen him too.
So its unlikely imo, Oswald came out JUST AS Baker was ascending the entrance steps.
Its more probable that IF Oswald and Baker met each other on the 1st floor,it was just AFTER Baker enters and that would place Oswald either in the front lobby storage room not in LOS at first, but then exiting from that storage room, OR, the 'vestibule" reference could mean the office area just after the 2nd set of glass doors, where Baker and TRuly had to get thru the counter and then Baker was surprised by Oswald exiting the Domino room as Baker and Truly went diagonally across the 1st floor towards the 2 rear elevators and rear staircase.
Otherwise, the "vestibule" must be refering to the smaller area 2nd floor space between hallway, landing and the lunchroom and office of 2nd floor.