I think it's kind of embarrassing that I did this to begin with LOL
But a small piece of plastic dropped into the dash vents - should I be worried about this? The last thing I'd want is a melted piece of plastic in there...

 

You're not the first or last.

Depending which vent it was, likely it won't even make it down to the heater box.
Likely it will just sit in one of the tubes.

I wouldn't worry.

How big was the piece and which vent did it fall in?

 

considering the entire hvac system which routes air is made of plastic, you'll be fine.

 

Thanks for the reassurance guys!
The little plastic piece came out after blasting the A/C