They are not venom injecting fangs per se. The venom enters the prey via a chewing action.
Their venom has nothing to do with the prey they eat. Their venom is not made of any alkaloid components - which are the main toxins in the amphibians that make up a part of their diet. Studies on heterodon venom have revealed that it is comprised of standard snake toxin families.
Hoggies make their venom in dedicated protein secreting glands - totally different toxin types and production mechanisms.
Keep in mind that there is a distinction between the compounds used by animals and plants as chemical defenses against ingestion by a predator (toads/frogs). These are simple chemical compounds which is in contrast to the more specific and complex multi-protein venoms utilized by snakes.