Vinegar is a natural mold inhibitors. I don’t know the restrictions on keto, but there’s several other ingredients besides vinegar that can be used to inhibit mold as well. These are referred to as Clean Label Mold inhibitors. The links below give an overview on Clean Label Mold Inhibitors.
If the keto restrictions eliminate use of these natural mold inhibitors, then freezing your bread will be the only alternative. All other mold inhibitors are chemical preservatives (e.g., calcium or sodium propionate; sorbic acid or potassium sorbate, etc.)