Cutting stainless steel vent chimney to fit sloped ceiling

You don't have to send this to a metal shop, I've DIYed these before with success although it's not for the faint of heart and it's an investment of time. These are decorative, covering up the extraction tube, they don't need to be airtight and they aren't structural. An angle grinder with a thin metal cutting disk is a good tool, but you could also use a Dremel or tin snips. Make cardboard forms which conform to the shape of the wall, then use the form to determine your cuts. Use masking tape to show the exact lines, then cut carefully.

Once you have the cuts you need to get the whole thing back together. Welding gives the nicest look, however you have to go very slow, doing a series of tacks and letting the metal cool between each tack or you risk melting it all, then you have to grind it back and polish it to make it look good. Lots of work. Brackets and metal screws are the easiest way to go, rivets will work too and give a flusher look than screws.

Remember with this sort of thing failure is always an option, so it pays to measure several times and make sure you have everything right. Also make sure you have enough of the section before you start as you will be chopping parts out of it, and make sure you've got the right tools and hardware.

If it is magnetic ( ferritic or martensitic) ,it will cut just about the same as steel,possibly a little stronger/harder. If it is mostly non-magnetic ( austenitic , 18-8) it may be tougher to cut with snips but will grind the same as steel. A very specific answer would require knowing the level of cold work in the vent steel, which makes any metal stronger.