Thanks for contacting Ask Extension.  Here is a recipe out of the Ball Blue Book for Vinegared Red Onions that are processed in a water bath canner.  It is considered safe. 
(Yield: About six 1/2-pint jars)

3 pounds red onions (about 2 medium)
4 cups red wine vinegar 5%
1 clove garlic
Ball Pickle Crisp (optional)

Peel onions. Cut onions crosswise into 1/4-inch slices. Separate onion slices into rings.
Combine vinegar and garlic in large saucepan and bring to a boil. Reduce heat and simmer for 5 minutes. Add onion rings. Simmer covered 5 minutes. Remove garlic.
Fill hot jars with onions and brine leaving 1/2-inch head space. Add 1/16 teaspoon pickle crisp to each jar if desired. Remove air bubbles, wipe rims and put on lids and rings.
Process in boiling water bath 10 minutes. At end of processing time remove lid and let cool 5 minutes before removing jars. Adjust for altitude over 1000 ft.

Good Luck!