Vergis never self-heals on his own. Do you mean why doesn't he self-shield? Or why doesn't he put continuous heal buffs on himself?

For OP, Vergis passive:

i

So, breaking Scarab's shield in and of itself isn't something that will necessarily proc the passive. So given what you've said, I'm not sure if it's an issue of a misinterpretation or a cooldown issue.