Vapour lock is seldom an issue with gravity-feed systems, since
the fuel is under positive pressure (though very low positive
pressure). Vapour lock happens most often when the fuel is beling
pulled rather than pushed, as in a low-wing with the tanks below the
engine's fuel pump. The lowered pressure on the fuel in the lines
lowers its boiling point, and the heat in the cowl does the rest.
Mogas in a gravity-fed system can be a problem at altitude,
since the atmospheric pressure is low, and the fuel's boiling point
comes down. If the engine is working hard and the ambient temp is high
enough, the heat in the cowl might raise the fuel to the lowered
boiling point. As the fuel boils, the vapours expand and prevent any
further fuel flow. Carburetors and fuel injectors don't work well on
vapours.
Mogas has a higher vapour pressure than avgas, which means its
boiling point will also be lower.

We used to run mogas in our O-200s and O-320s. The
Continentals didn't like it, and the valves would stick and the valve
seats would wear out sooner. The old-technology bronze valve guides
relied on the lead for lubrication, and so did the valve heads and
seats. The Lycomings gave somewhat less trouble, but we still had
exhaust valve guide wear and valve burning problems with it. Whwn the
guide wears, it lets the valve hit the seat a little off-center and
makes the seat wear to an oval, allowing it to leak and burn. The
problems went away when we went to straight 100LL.
Today's automobiles have much more advanced valve
technology that allows them to burn the harsher mogas. The
ContinentoLycosaurus prefers the diet it's been on since it was born
in prehistoric times.
Running a mix of 100LL and mogas didn't work much better than
straight mogas. Sometimes these economy measures are false and end up
costing more in the long run. We run a flight school, and besides the
cost of expensive cylinder repairs, which is bad enough, the downtime
eats into the budget real quick. 100LL is much cheaper for us.

Dan