50cc Yamaha Vino bogs down at 10-20mph

[cheapeto]

I do not know your machine, but those 2 strokes can fly man.
You may need to get into the variator and clutch assembly.
I once put sliders in wrong, and the result was I could not get my 250 cc motor past 40 mph.
Also once when I installed a new belt, I had to ride like 10 miles before it seated properly in the variator.
You may want to blow out the clutch bell while your in there and check your final drive oil, as many types of scooters you need to fill it after your remove the variator cover.
Good luck.