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IMO the temperature difference is so tiny it won't matter in the least bit. Basically, the theory is that during the day it gets warmer, so the gas you pump into your tank is warmer and less dense. So, you get more gasoline for the same amount of money. I don't think this theory is very sound as the gasoline is kept in tanks that are underground. There is no way that tank heats up much at all during the course of one day. Week to week maybe there is some variance, but not day to day. Also, the time it takes to pump the gas from the tank to the tank of your car is not much time for it to warm up and expand either, and thats not to mention that the lines are pretty well insulated.
As for the vapor, you are correct. Modern vehicles have vapor recovery systems that feed gasoline vapor from your gas tank to the intake manifold to be sucked into the engine and burnt. Everytime you open up your tank to the atomosphere you will loose some vapors no matter what. But, the amount is so small you will never notice. I don't see how filling up at less than a half tank would make much of a difference other than allowing more area for vapors to occupy. Since they are burnt, its not an issue.
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