Does the mountain shape have special significance? You could probably lay the strips out on the floor and draw the shape you want and just cut it out. The shape of a house might be cute to draw people living and doing things in it. I also think a big rectangle and some chalk are enough to get young imaginations going.

Is it hard to cut the vinyl? If so, you could put up the sheet as is, and create the mountain outline OVER it with, say, foam core board or the like. You could even change out the shape using different foam core board later. You could make "shutters" so that the chalk surface is a "window" where she can draw a view, or a "stable door" if she likes to draw horses, or a house where she can draw the inside, or shape it like a guitar or piano. The vinyl sheet stays the same, it's the surround that changes the shape.

There are lots of people on Etsy that sell vinyl wall decals. I can’t imagine that you couldn’t get someone to do one in chalkboard vinyl. If you don’t see one you like Etsy sellers usually are happy to do custom.

or if you’re set on diy, a search for “mountain vinyl wall decal” could be a good source of ideas.