Because of your other post concerning your attic issues, I would say go ahead and use the insulation you bought. It won't catch fire. If the vent got hot enough to combust anything, then your clothes would catch fire if you got anywhere near that stove.

But, the insulation is not going to do you any good if you don't seal the vent pipe, and the insulation will not provide that seal. What you need is vent mastic. Its a putty like material that you paint on all the seams in your vent. You use it where the sections of the pipe connect, the seams of the curved sections and the seam down the sides of the pipe where it is put together. Then you can put the insulation on over that if you like. I don't know if the insulation will help, but if you put the mastic on first, it won't hurt.

If you put the insulation on without the mastic, it might make matters worse. It would trap moisture that escapes from the vent and hold it against the pipe, causing it to rust out in a year or two.

BTW, do NOT use duct tape for this.