Vain Imaginings are those useless thoughts that go through a persons mind when they are being useless. The term is commonly used in religiousdocuments. That's because God doesn't want people sitting on their ass doing nothing all day.
Pre-imagizing is when someone attempts to explain directions or a situation using information only the person speaking would know.
Tom decided to pick up three of your "old friends" from the local bar before grabbing you at your job at the west side diner. He left a voice-mail saying he was with your old softball team, but they were all strangers. Tom is always pre-imagizing stories.