I usually think it's kinda dumb to say, this guy is a top 6 guy or bottom 6 guy and can't really be the other. Vatrano seems like the type of player who that actually might be true for though. He's a really smart player who can shoot the puck -- I think pretty soon he'll be someone who can score a ton of goals with someone like Krejci or Spooner and a winger who can drive possession. And it's really valuable to have players like that, who might not be one of your top 6 forwards in a vacuum, but can thrive playing a specific role on a 1st or 2nd line.
But right now he's not ready for that kind of role. Even with his improved conditioning, he's not great defensively and can't consistently create his own offense at the NHL level (like Marchand could at the begining of his career), so he's poorly suited to the 4th line. He might be able to work with Spooner, although a Beleskey-Spooner-Vatrano line is terrifying defensively