Reason 1: Cigarettes had boundaries. Your vape doesn't.
Think about how you used to smoke. Even if you smoked a lot, there were still little barriers built into the act. You had to get a cigarette. Find a lighter. Light it. Smoke it. Finish it. Put it out. And depending on where you were, you might have needed to wait until you could go outside. There was a beginning.
There was an ending. There was a moment where you could say:
"That was one cigarette."
Your vape changed that. There is nothing burning down. Nothing telling you that the session is over. Nothing stopping you from taking one pull now... another two minutes later... another while answering a message... another while standing up... another when you sit back down.
The convenience that made vaping feel easier can also make the habit far less visible. One former smoker described the problem almost perfectly:
"Now I can just hit the vape non stop."
That is the first reason it can feel so different. With cigarettes, the habit happened in episodes. With a vape, the episode can quietly stretch across the entire day.