"Styrofoam Sounding Squeak"

Youre not alone - in fact almost every Prius has those infamous squeaks at one point or another. Sometimes it happens, sometimes it doesn't. But usually it happens in colder weather when the plastic shrinks and has more room to rattle.

My squeak was coming from in between the drivers' dash and porthole post (triangle window). I created a shimmy out of a gray piece of foam I had lying around, to jam in between the crack where those 2 pieces of plastic meet. Luckily it matched my car's interior and its worked ever since. I didn't need to take anything apart to do this either. I still get squeaks and rattles from my rear hatch and other random places but I can live with it.

It might be hard to locate where the squeaks are coming from - sound is tricky and will bounce off the car in random areas, throwing you off. But the main problem areas most people have had seem to be the dash and MFD as you also mentioned.

You say the squeak might be coming from inside the vent, but I think it's external- most of the Prius plastic is very flimsy. To target this area, try to have someone hold down the plastic you think thats squeaking while youre driving to see if it stops squeaking.

 

I have an 07 as well, and I have nearly the exact squeak you mentioned (though mine sounds a little more like plastic rubbing together).
Best I can tell from my limited experimentation is that it is the clear plastic covering the dash display. I looked at the possibility of it being the lens where the speedometer and such originates from, but that seems pretty solid.
The clear plastic on the dash where your speedometer and notifications are is actually two pieces...the only way I was ever able to get it to stop squeaking at all was by putting pressure next to the area where that 'split' was (some people have mistaken it for being cracked). This temporarily stopped it, and now it's intermittent. I have yet to figure out what I'm going to do to fix it completely .