Glass tables don't need "protecting". People sometimes put glass over wood to protect the wood. any table top will be dinged by dropping things on it including anything you might put over top the glass.

I had a glass topped table on my patio fors everal years, stayed out there allw inter through snow ice etc. Bever got a scratch....but Iw as tired or trying to keep it clean in the summer.

Hard water rings might be an issue for vases that are placed in the same position regularly. Be sure bottoms are dry and use runner or other barrier.