I've not checked the stats for the forwards you've quoted but for Costa and Kane, because they play for the two teams fighting for the title and that's what United should be doing.
While I don't think Vardy would've changed our season, based on Costa and Kane's output, you may well have a point that Ibra's relatively low conversion rate is the reason we're not in the title race. Ibra has scored 17 Premier League goals from 115 shots (
link). That rounds to a chance conversion rate of 15 per cent as you say. By comparison Diego Costa has 17 goals from 93 shots, which rounds to a chance conversion rate of 18 per cent (
link). Harry Kane has 20 goals from 76 shots, which rounds to a chance conversion rate of 26 per cent (
link).
Its not just about shots either, its the type of shots Ibra's missing that's been an issue too. In the league this season Zlatan has missed 18 'Big Chances' (chances that you'd reasonably expect a player to score). That is double the 9 Big Chances Kane has missed, and nearly double the 10 Big Chances Costa has missed.
The last time we won the title Robin Van Persie scored 26 Premier League goals from 141 shots, a chance conversion rate that rounds to 18 per cent (
link). As Costa and Kane are showing, that 18 per cent is the benchmark for a successful title chase.
Zlatan has been an amazing player for us, the scorer of some superb goals. Yet, he has a tendency to bury the sublime and miss the ridiculous. His opener against Sunderland an example of the former, the free header he missed between the Everton posts a few days before an example of the latter. Sadly for us those misses have added up. If our main striker's chance conversion rate was the same level as Costa's, the same as Van Persie's in 2012/13, we'd be fighting for our 21st title right now.