Vulcan/packages/vulcan-users/lib/server/create_user.js

import Users from '../collection.js';
import { runCallbacks, runCallbacksAsync } from 'meteor/vulcan:lib'; // import from vulcan:lib because vulcan:core isn't loaded yet

function createUserCallbacks (options, user) {
  user = runCallbacks("users.new.sync", user, options);

  runCallbacksAsync("users.new.async", user);

  // check if all required fields have been filled in. If so, run profile completion callbacks
  if (Users.hasCompletedProfile(user)) {
    runCallbacksAsync("users.profileCompleted.async", user);
  }

  return user;
}

Accounts.onCreateUser(createUserCallbacks);