How do I change the spark plugs in my Yamaha Vision

This could also be caused by a faulty plug cap or coil.

Seeing that the coils are much easier to test than the carbys are to remove,strip,clean,re-fit,adjust mixtures & syncronize,I suggest you start there. (see my tip on how to test motorcycle twin coils-in tips & tricks).

Just to get an idea what is going in you could also try & remove both spark plugs & compare them to each other.
Is one black & sooty? - too rich- poor combustion
Is one oily?-- not firing or mechanical problem
Hope this helps
Regards Andrew