I've had some hesitation with my VXs as well, one thing that helped me was that I noticed when filling up that the gas tank had vacuum. (big hiss when unscrewing the cap) Replacing the gas cap cured some of the stumbling. It's easy to test by leaving the cap fairly loose and driving around a bit.
Yes. I have gotten almost 15 gallons in my VX also.
The second VX I have doesnt get more than 10 though.
I also found out somewhere that some VX tanks were 14 gallon though.
I also found that NON Ethanol gas give you about 10-15 percent better MPG.
And its true that when the pump shuts off, most of the time you can get another 3-4 gallons in it. So that is important for mpg calculation to fill it until it starts to come out.
Also, I found that if the car is on a hill tipped away from the gas pump, that when filling it up you can get the almost 15 gallons in it.
Hard to find pure gas for cheap around Tallahassee though.