Modern Vespa

Does anyone ride sitting all the back on their seat? is this dangerious?

I don't do that, but it shouldn't be inherently too dangerous... it is best not to put weight beyond the rear axle, but Vespa seats don't really allow this.

I think it is best to have your weight closer to centered between the axles and low, which means sit more forward. Less aerodynamic this way, unless you really hunch, but I think it is best for traction and stability. However, as I said before, I think it is most important your weight doesn't pass the rear axle.

I only do that when laying flat while bombing a downhill. My legs are so short its the only way I could do that. It gets me that extra speed I need

I do this frequently. It gives my legs and knees a chance to stretch out and the sportbike position varies my riding position enough to ward of riding fatigue a little while longer.

i like to vary my seating position especially on long rides. i had the strap taken off my seat so i can slide back and stretch my legs when i need to.

I usually sit at the centre of the saddle (6 feet tall), but have sat right at the back when racing smaller scooters. With the speed of a GTS, you have to worry about not having enough weight on the front wheel for stability.

Mike

I had a Corbin seat put on my GT200 and essentially eliminated the seating area for a passenger. The seat now makes it easy for me to ride in the "traditional" position but also to slide back quite a bit so I can vary my riding position on long rides or in response to wind and road conditions.

I also have a friend who rides back on the passenger seat of his vintage Lambrettas and has been doing that since they were new Lammys in the '60s with no ill effect. (In fact that's him, sitting on the back seat of his Lambretta back in the day in my current avatar.)

That said, I have a topcase and even with the ultra-heavy bar weights if I don't pack the scooter carefully for trips and end up with too much weight in the back there can be a notable "floatiness/wobble" with the front wheel, resulting in an immediate stop to redistribute the weight and put everything heavy into the pet warmer.

All of this is simply to say that you'll want to take multiple factors into consideration relative to riding position and adjust accordingly as you ride. If you feel safe, stable, and in control, then you should be able to sit anywhere on the seat you wish.