[MHT]

Is there a way to counter the vapor? Add more fuel before flying? Run the plane more minutes before take-off?

[MisterEd37]

Part of preflight checks is to drain a couple of ounces of fuel from the lowest point in the fuel lines. Water is denser than 100LL, so you get the water out of the line and all is good.

Most light plane crashes are due to overlooked checklist items and poor planning.

[Falcon4.0]

Yes, you are right. My instructor taught me to shake the wings to gather the condensed water from the top interior of the fuel tanks and wait for at least 30 minutes before checking the fuel taps for water.
There is also a fuel bleed lever in the engine compartment. Unfortunately some pilots are eager to takeoff and overlook safety.
My instructor also warned me to smell the fluid in the sample cup as it's possible that it could just be water.
Most times pilots just look for little beads of water at the bottom of the cup not realizing they have a full cup of water.