Cutting curves in sheetmetal is a good job for a pneumatic nibbler. They can cut strait lines or curves with any radius and can be a little cleaner than a jigsaw because they support both sides of the material while punching out little bites.
If you don't have a decent air compressor, then they also have electric versions and even drill attachments that do ...
If the only tool you have is a jigsaw, then make a radius guide. You'll need a fine-tooth metal cutting blade, and maybe several. A sabre-saw is not suitable for this approach.
Start by drilling a center hole in the van, and carefully measure inside and out to make sure you're clearing any framing members or reinforcements. Presumably you've removed any ...
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 ...
How about instead of a single potted plant, you have either:
a) a series of potted plants that take up the entire platform or
b) a carpenter builds a custom rectangular planter that takes up the entire platform.
Plants always do a space some good especially with all that natural light.
Aviation Tin Snips are also an option, however being manually powered they're a lot of effort.
If you're right-handed then you probably want the red-coloured ones, because they turn to the left as you cut. Left-handed users would want the green-handled ones in this instance.
Downsides are that you need a fairly large entrance hole, and like in @harper's ...