For those concerned about the formation of vapor in a pressurized fuel rail, I've attached a plot of data collected from local samples of 100 LL avgas, 87 octane auto fuel with no ethanol, 91 octane auto fuel with 10% ethanol, and tap water.  The water was measured just as a check on the method.

 

The data would be considered more of a true vapor pressure rather than a Reid vapor pressure due to the method used.

 

The data indicates to me that if the fuel pressure in the fuel rail is 35 psi as measured with a regular gauge referenced to the atmosphere at sea level, the temperature of 100 LL or 91 octane 10% ethanol in the rail would have to be in the neighborhood of 240 deg F for it to form bubbles of vapor (boil).  The sample of 87 octane would require a temperature of about 215 deg F to form a vapor phase.

 

My take on this is it may be more productive to be concerned about the fuel supply to the high pressure pumps rather than worrying about "vapor lock" downstream of those pumps.  This seems to be the conclusion reached by the recent thread on this subject, possibly now supported by actual data.  Of course the data only applies to the samples I obtained.

 

The higher temperature tolerance of the auto fuel with ethanol compared to the auto fuel without ethanol was surprising to me.

 

But I only collect data --- it is up to an engineer to make sense of it ;>)