“The much-anticipated book ‘A Warning,’ reportedly written by an unnamed senior White House official, claims that high-level White House aides were certain that Vice President Mike Pence would support the use of the 25th Amendment to have President Donald Trump removed from office because of mental incapacity.”
(NOTE: The author of this book is the same person who gained notoriety for an anonymous NYT op-ed some time ago.)
“According to Anonymous, there was no doubt in the minds of these senior officials that Pence would support invoking the 25th Amendment if the majority of the Cabinet signed off on it.”
This is basically gossip at this point. The author does not specify which aides believed that, or how many, or why. It is possible that this was just two people stating their gut feel, and did not involve any overt indication from Pence. Whether the officials believed it or not, I see no evidence that Pence did anything to create this belief.