Question: "Each time he made the turn, he saw only the white, unused, buckling sidewalk, with perhaps, on one night, something vanishing swiftly across a lawn"... p.11 Fahreheit 451

Does it mean that the sidewalk is bent (upward), as when a tree grows next to a sidewalk and distorts it to make room for its roots? But then again, it seems paradoxical that it's buckling but also unused... Any thoughts?

Answer: 
"On vacation" is an *adjective phrase* describing Susan's current state. "Go on vacation" is a *verb phrase*. They're different parts of speech, which makes it difficult to compare them in any meaningful way.

Perhaps some similar example pairs will make it clearer:

"*fall asleep*" and "*asleep*"  

"*get on a train*" and "*on a train*"  

"*sit in a chair*" and "*in a chair*"
"On vacation" simply means "not at work" or "not in school", etc. - you're taking a break from your usual duties. If someone is on vacation now, it simply means they're taking time off. Some people travel for their vacations, but not everyone does - in fact, there's a colloquial term "staycation" for those who *stay* home rather than go somewhere else.

I agree with Kate Bunting that "going on vacation" may imply traveling, but in the end it really is a question for Susan ;)