Try stuffing it in there, it looks like it will fit from the inside, if you use a regular jigsaw saw, it will crack, tear, and be uneven, go buy a cheap 20 dollar dremel if you can, they will do it....luckily i bought one back then
you got a vice grip? take a towle and set it in the vice. next place the visor in (make sure it does not touch bare metal to avoid scratches). third snugly tighten the grip being sure not to "crush" the visor. next use a hand saw. not a power tool as in this position it would snap it.
ok now saw it slowly making sure to brace it with one hand and saw with the other. it should not break and should be fine this way.
The fact your forcing a wedge of metal between thick plastic wont work, it will crack, I've had this happen to me many times before in the past and wasted several sheets and pipes, the best it so have something electric like a dremel.
Go look for a pvc saw, it's got very small tiny teeth similar to a dremel blade, it is still risky, but I doubt it is jagged eniough to rip the visor into two peices, it will not do the smooth job, but it will take off the part you want gone.
The visors are very flexible, you should be able to bend it and put it in the bottom of the helm, don't try to put it through where the visor is suppose to be. And you will need a dremel so I would just buy one now, rather than later.
May dont need a dremel if you dont have one, a set of dremel heads from the dollar store will work fine on a drill, just use the heads as a drill bit, plus a drill will always have more power, just thought of that as a alternative. a set of heads cost 5 dolalrs at big lots.