can this product be used for weed killer?

If you add 2 gallons of water to it, you will have 11% which is in the range 10-20% recommended. The standard store vinegar is too weak. Then add 3 cups salt, 3 TBSP's dish soap, and you are all set. Use a sprayer or a plastic watering can. Wait for a sunny day with no rain in the forecast. Wear protective eyewear, skin protection, and rubber gloves, rubber boots, etc, don't breath in the vapors, and hold at arm's length when pouring the bottle into a bucket. If you want to plant the area, use orange oil instead of salt, with same ratio.

Do all these industrial vinegars have to be mixed with orange oil as an herbicide? Some seem to use dish soap.

IMPORTANT: If you are using this mixture with salt, make sure it isn't on a flower bed or a potential garden area because the eventual salt buildup inhibits the the growth of plants and flowers. I recommend just forget about the salt and just pick a dry sunny day to apply the vinegar and dish soap combo. Dish soap helps draw the vinegar down to the roots.

If you apply it correctly to weeds but without salt (on a sunny, dry day) will the weeds come back? In other words, does it also kill the roots?

Add a little dish soap to help the vinegar penetrate the soil. It will kill the roots.

Does anyone know if this vinegar will destroy the rubber fittings in a weed sprayer wand?

Any 30% vinegar will eat thru the rubber seals in a Chapin style pump sprayer if it’s not properly cleaned after use. That means dumping the tank and triple rinsing with clean water and flushing the wand as well.