Capturing vapors in a portable gas can is idiocy at its best, it just doesn't happen.
This is quite simply wrong.
The lack of a vent on portable gas cans is more about preventing the escape of vapors in storage, than about capturing vapors displaced by the dispensing of gas.
Venting the cans causes four big issues I can think of, off the top of my head.
Did you realize that gasoline blends can contain 15% butane nowadays (ok, only in winter blends, but still).
It's in solution, but it will fraction off just sitting around. So the first issue, is if you leave the can vented, it will be 15% (or more) lighter in a few weeks.
This loss of fuel costs you money, but more importantly, it also lowers the octane rating of what's left behind (that's issue #2). This is counter-intuitive, as you'd think that the more volatile fractions would lower the octane, but butane has a RON rating of 94.
This combined lower octane and volatility makes starting harder (but prevents vapor lock).
No amount of gasoline stabilizers will help with these issues, but I guess if you're willing to pay close to double the normal price for gasoline (and deal with the issues introduced by lead), you could use 100LL aviation gasoline. It's low volatility (necessary for operating at altitudes with reduced air pressure, to prevent boiling) would mean that the pressure in the can's headspace would remain low (which would mitigate the next issues too, and which is why aviation gasoline stores much better than car motor fuel).
Gasoline in a vented container "breathes", and the more volatile the fuel, the more it breathes. As the daily temperature swings, the pressure in the container's headspace will go up and down. It vents volatiles during the day, and ***** in air at night.
This exposes the gas to fresh oxygen, which accelerates the process of producing varnish (issue #3). This ONE issue can be prevented by the use of gasoline stabilizers.
The ethanol in blended gasoline is hydroscopic. That means that it will literally **** water out of the air. This isn't so horrible, so long as the ethanol/water mixture stays in solution in the gasoline, but with gasoline being a primarily non-polar solvent, and water being very polar, it doesn't take much water in that ethanol, to get the solutions to separate like a salad dressing badly in need of shaking. This water/ethanol/gunk mixture will rot out the bottom of your tank, but more importantly, causes havoc if it gets into your engine.
Some gasoline stabilizers help with this too. Either by preventing the "phase separation", or by mitigating it's effects by blocking the corrosion caused by the water.
Now, if your gas can is not vented, it won't breathe, and the liquid will not be exposed to any more water vapor and oxygen than what is already in the headspace (this is very little). Problems all avoided.
So long as you don't have winter blend gas stored in an unvented can left out in the sun in the summer, it may look like it is unnaturally expanding, but it will not explode. Today's cans are pressure rated for something like 10PSI (above which, the valve is supposed to vent before anything fails from the pressure), and that is a lot of pressure for a small plastic can, so it can be expected to expand a lot.
