So on the first floor (the street/concourse level) of 6 WTC, there weren't any stairs or elevators that took you to the second floor (the plaza level)? That's strange, I figured there would at least have been stairs or elevators in the parking lot area on the first floor that took you to the second floor.Yes probably the Vista/Marriott hotel also had its own parking garage too and it had its entrance right next to the main entrance to the building on West Street if I'm correct, they also should have access to those parking garages by elevator. As for 6 WTC, people only could access the building from the plaza level as the main entrance in general.
Here's a map of 3 WTC's lobby on concourse level:
I don't think there was an exact parking lot on the concourse level of the hotel. But I do think the hotel's lobby had elevators and stairs that took you to the underground parking garage on the B-Levels.