The question is mostly for VX owner, but in same time to everyone.
I do run Supreme fuel in my VX since I own it (91-92 octane). Where I live, there are lost of hill to climb and since I don't want to downshift, I put supreme fuel so the car run better (especially at low rpm because VX does run often below 2000rpm). Also, since I'm crussing 60mph (100km/h) on highway, I'm telling myself that I have a better FE with supreme fuel.
1) Premium costs more - sometimes much more than regular 87 octane. Kind of defeats the purpose of buying a car that gets good fuel economy.
2) Higher octane fuel is more difficult to ignite than regular gas and it is formulated to be more uniform - it contains fewer volatile components that burn at lower temperatures. High performance engines run hotter than econoboxes so it is more critical for the high output engines burn gasoline that won't pre-ignite or detonate. So, if you decide to drive your VX like someone delivering pizzas in 10 minutes or less, you might need premium. If you drive like your grandma (that's my driving style) you don't get any benefit from premium at all. That's the short answer.
My VX's have not run on anything but 87 octane except when my current VX's engine was overheating due to a clogged radiator. I put in mid grade 89 octane just in case the engine was reaching the point of irregular combustion with the 87. I did not need to use 89 octane for more than one tank full and only as a precaution - I replaced that radiator quickly! No blown head gaskets for me thanks.
My arguments (that I gave to myself...) was that when I was accelering at 1700-1800 rpm to climb a hill, it was better for the motor with high otcane fuel, and I thought I had a greater MPG to, but I never REALLY do any test. By swithing to 87 octane, I will see if the last arguments stand or not, I doubt it tho. Thanks for the fast reply.