You can't cut a circle in a van side.
A circle would require the van side be flat. It's not flat, it's curved. The curve gives it a great deal of strength. If it wasn't curved it would go "waggle-waggle" as you drove down the road. The shape is akin to a Pringles potato chip.
If your window depends on being a true circle in a flat medium, and not a Pringles chip, then the window will not fit.
How to cut it, though
The van metal is VERY thin - its strength comes from being stamped in a curve, not thickness. The #1 problem will be distorting the outside of the circle and creating yourself either ugliness or a need to do body work and paint. Almost any power tool will tend to do that, as will many hand tools.
With that in mind, I would tend to go old-school. First, as JACK advises, lay down masking tape and accurately mark the circle precisely on the tape. If you bodge it up, peel the tape, lay new tape and mark again. That's cheap/easy.
From there, I would use hand tools, such as a VERY fine tooth hacksaw, working carefully. Start it by drilling a hole well inboard of the edge. Either use the kind of hacksaw where part of the blade extends beyond the tool (and don't kink it).... or, cut a 4" diameter hole in the middle using coarser methods, cut the first inch with the bare blade (wear gloves, I show this in red) and then put the bow of the hacksaw through that hole. 12" is a large enough diameter that a hacksaw can make that curve. You'll have 45-ish lineal inches to cut, but that's not so far in such thin metal.
If I had to use a power tool, I'd use a Dremel with the little cutoff discs (you know the ones, they come in a 40-pack typically). The Dremel is too small to distort the sheet metal, and as always with those discs, they shatter when you are not careful, and patience wins the race. A hand hacksaw would be faster, but some people need to use power tools.
Make sure not to leave the masking tape on for more than needed, and cover it well if you must leave it. Over a few days, heat of the sun, and UV light from the sun, will damage the masking tape, and the damaged adhesive won't peel, and will leave very stubborn gunk on your paint job.