What is the best way to seal the gap between the wall and the vent?
The first thing that comes to mind is Styrofoam. But then the pipes are stuck forever. Are there special solutions for this? Expandable tape that you squeeze into the gap, or can you tape the gap shut? Can you tape it over the pipe and wall?
The back of the hole goes out into our garage. That's where I put acrylic on everything and painted it.
A compression strap, of course, is possible if you're nimble enough. It usually covers the gaps. It would also make sense to use it on the outside of the acrylic.
You can't push the strap in too hard, it should stick to the pipe and then expand. Alternatively, you could choose a round cord that is wide enough to fill the gap well, and then tape it from the inside and paint over it.
You could also spray cartridge sealant into the gap and then treat the joint with round cord and acrylic paint to give it the look you want.
I would push loose insulation wool into the gap and caulk the very front gap with acrylic.