Flood Vents and Raised Gardens
As long as you don't block off the vent at all, you should be fine. I'd get a picture if I could, but I had one customer who had a very similar problem to yours, and because the raised bed was higher than the top of the vents, he built a slanted box, about 55 degrees, from the bottom of the vent up over the soil line. It was basically a 2x4 box covered in plywood, but it looked great.

As long as you don't block off the vent, I don't think you'll need a permit or anything for a project this size, but I am not the expert on that area. If you build something, make it sturdy enough that if it does flood, your structure will remain intact and usable.