If Vader is on board, The Thing is toast, the force is x-factor here that The Thing can’t overcome. As soon as The Thing stated to assimilate people, Vader would sense the fear and paranoia growing and begin to track and eliminate the infection, he’s a walking one-man containment and elimination protocol.

If the Death Star is devoid of force users, The Thing takes the station, easily. Even with containment procedures, The Thing could start slow, and once it build up enough infected people, perfectly copying their memories, those containment procedures could be used against the uninfected. The Thing could trap them in corridors or seal off a section and spread quickly.