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Temprature:
What I forsee happening by adding gasoline vapor is if the air temperature isn't high enough to keep it vaporized it will condense back into liquid droplets. But if the intake temperature is high, the chances of detonation go up meaning you have to lower your compression ratio and/or retard the timing, both of which will hurt fuel efficiency.
And even if you could get it into a gaseous state in cool air, the liquid fuel droplets act as a way to cool the charge to prevent detonation. So again, you'd have to lower the compression ratio or retard the timing or use higher octane fuel. This is why natural gas is a good fuel for this since it's already a gas and it also has an extremely high level of octane.
One drawback to gaseous fuels is you get less power. But that can be a good thing for fuel mileage. Since gasses take up more space than liquids, you get less air into the cylinder, and therefore less fuel to maintain a correct AFR. That also means you have to keep your throttle more open, resulting in less pumping losses.
But what could be helpful is extreme atomization. If you could make the fuel as fine of a mist as possible that would help on most all accounts. Maybe gasifying gasoline and feeding it into air that cools it into a fog-like mist would be the key. Or just finding other ways to get it as fine as possible. Ultrasonic vibrations? Of course avoiding things that deatomize fuel are just as important. Making the intake stream bend as little as possible with as few vortices as possible so that fuel doesn't get flung out of the air, for an example.
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