Poor defending saw Vale throw away a two-goal lead at Northampton despite a 30-second opening goal by captain Doug Loft.
Micky Adams brought Chris Lines, Adam Yates and Billy Knott into his line-up with Griffith, Birchall and Robertson among a strong bench.
The Valiants got off to a dream start when Doug Loft latched onto a Chris Neal ball and put Vale ahead after just 30 seconds. Coventry countered and had a number of early chances themselves but keeper Neal was once again in good form.
Vale had a decent chance for Lines from a free-kick but Coventry right-back Cyrus Christie should have equalised. But despite the home side having chances to level it, it was Vale who scored again when Myrie-Williams headed the ball towards goal and it looked like Ben Williamson got the final touch. It was the striker’s fifth goal of the season.
At that point, despite Coventry’s dominance, Vale looked in control but with ten minutes left, Jordan Clarke got one back for the home side after poor defending by the Valiants. And the Vale threw away another lead as yet more poor defending let in the dangerous Callum Wilson to score a 90th minute equaliser.
To be fair, it was what the home side deserved after having the majority of play as once again the Vale were left to rue yet more defensive errors.
Vale: Neal, Yates, Grimmer, Duffy, Dickinson, Myrie-Williams, Knott, Lines, Loft, Williamson (Griffith), Pope
Subs: Johnson, Robertson, Jones, Dodds, Tomlin, Birchall
Attendance: 1,627 (345 away)