The Vice President is not a cabinet member. He/she is the only other nationally elected official, the president of the Senate and, of course, first in line of succession. Many presidents also make the VP the head of the National Security Council and give them other high-level duties.
All of what you say is true. However, we will never know what sort of responsibilities or level of trust McCain would have given to Sarah Palin. All we have to go by is McCain’s erratic behavior since losing the election.