Question: In the last part of the movie, how are they supposed to drive the snow vehicles? There is a scene before where they cut the wires of all the vehicles to disable them for the quarantine. Am I missing something?
Question: They find the thing trying to escape the initial crash but it froze. In the 1982 version, they find another thing in a similar manner. Does this mean Kate was infected but didn't know it but turned as she walked off into the night?
Answer: In the 1982 version they never find another thing in a similar manner. It's just the one creature. Whether Kate was or was not infected is irrelevant, she was not found in the 1982 plot.
Chosen answer: By disabling the snow vehicles, they are making sure that the creature can't use them to escape from the quarantine zone. Since the creature can assimilate anybody, it would probably know how to use one.