With all due respect (and I sincerely mean that I do have respect for LEOs), it's not your place to have to explain to them what something is. Inhaling nicotine is not illegal, whether you're doing it out of a pipe, a cigarette, from half a coconut filled with egg whites, or from some funky apparatus that looks like it was made by a glassblower with a bad case of the hiccups.
Instead, just tell them the truth, being kind and patient. Like many adults, you enjoy inhaling nicotine and find vaping to be more enjoyable than smoking, and this device is used for that purpose. If they are still suspicious, chances are you'll have some juice in it and that's a perfect opportunity to negate the officer's suspicion. As many officers carry kits to test for known narcotics, you might ask if they'd be comfortable with testing a sample of the juice that's in it. That should be enough to assure them that while your tastes in nicotine absorption might be unorthodox, you certainly have nothing to hide. And if they DO actually test it, it'll just back up your story.