Ive watched a few videos on swapping out the flash hider on my AR. I really dont have a vice or any way to properly secure it:

can I over torque something or damage something by just trying to twist this flash hider off with the gun between my legs?

Doubt you'd be able to remove it. Actually depends on whether it's a crush washer, peel washer or loctited in place.

If you don't have the tools, find a local gunsmith to swap it out

^^This

I could get it off for you, but you're a fer piece down the road.

Does it have a front sight on the barrel, like military M16s?

If so, lay the rifle on the floor on it's side, sandwich the sight between two pieces of 1x4, and kneel on the top piece, then try loosening the flash hider... You won't be able to apply enough torque to hurt anything... The cross pin on the front sight is stout.

If it's a plain barrel, you can try sandwiching the barrel the same way (in front of the hand guard), but you may need a rough piece of leather on the barrel as well to keep it from rotating.

If it won't loosen before the sight lifts off the bottom board or the barrel rotates, then you'll need better tools or a friend who has them.

You can give it a try as you mentioned but, if it does not come off with just a little/moderate effort, I would then take it to a gunsmith.

Typically, muzzle devices are torqued to 8 ft.-lbs. Turn it with a wrench across the flats while kneeling on the receiver. It should break free almost immediately unless it is held with Rocksett.

Ill try it lightly and see what happens if it is tight I will send it to my local guy.

Lightly is the opposite of what's needed.

Get the action held tight, get a wrench on the flash hider, and knock the hell out of the wrench with a hammer. Shocking the tight nut or bolt is often what's needed to break it loose, and is actually easier on all the parts.

Who manufactured this upper? What caliber? Does it have a crush or peel washer? What diameter threads?

This is what I've always done with factory installed muzzle devices and they've always come off.

An AR vise block can be had cheap, no?

You aren’t going to hurt anything if you hold the rifle between your knees and put a wrench on the flashhider.

Or, buy the flashhider from a local shop that will put it on for free.