More than 1,500 Saddlers fans made the trip to Vale Park for the first meeting of the two sides since Clarke upped sticks in February.
And they could have hardly wished for more, with Matt Taylor's lot putting in arguably their best performance this season.
A near-flawless first-half saw Walsall show bucketloads of commitment and, most importantly, quality in attack.
Kiernan rounded off a lovely move by guiding a low shot into the far corner – and that proved to be the winner.
The Saddlers were not quite as free-flowing in the second period, but the application never wavered.
Vale skipper Tom Conlon would then get sent off after two bookings as the visitors professionally saw things out.
This statement victory sees Walsall move up to 11th in League Two.
Having beaten Clarke, Saddlers supporters will be on cloud nine. A thoroughly-deserved three points.
Walsall made two changes from their 0-0 draw with Rochdale at the weekend.
Hayden White, having served a one-match ban for five yellow cards, returned at right-back. Otis Khan dropped to the bench.
The Saddlers' other switch saw Kiernan replace Tyrese Shade at left wing.
Vale, meanwhile, had joint-top scorer Jamie Proctor among their substitutes again after injury.
Saddlers supporters went into this clash with bated breath, desperate to get one over Clarke, and they made plenty of noise in the build-up to kick-off.
On the pitch, Walsall made an encouraging start as well.
As the travelling faithful made their feelings towards Clarke abundantly clear, Conor Wilkinson tried his luck from a free-kick.
Lucas Covolan in the Vale goal fumbled it but, just about, gathered at the second attempt.
A nice spell of possession soon saw the ball worked to George Miller, whose half-volley flew a few yards over the crossbar.
Stephen Ward also surged forward and won a free-kick in a dangerous area, and while Jack Earing overcooked the delivery, the Saddlers were going about things in a positive manner.
Centre-half Rollin Menayese picked up an early booking for hacking down Vale forward David Amoo, but, on the whole, they looked measured and were doing a fine job of frustrating the home crowd.
It was a very strong first 20 minutes. Walsall were pressing high and battling well, and they continued to put the home side under pressure.
Wilkinson worked himself a yard and from an acute angle, he was denied by a combination of Covolan and the post.
Then on the other side, Kiernan saw his toe-poked attempt turned away for a corner.
The Saddlers were certainly knocking on the door and just shy of the half-hour mark, they got the opener they deserved.
Manny Monthe needed to be on his mettle to slide in and stop a Vale counter-attack before a swift move finally broke the deadlock.
Wilkinson played it through to Kiernan, who cleverly escaped the offside trap. The winger could have slipped in Miller but decided to go it alone and found the bottom corner.
Monthe had to be sharp again before the break as he raced across to deny Amoo.
As far as the half-time team-talk was concerned, though, Taylor would have simply asked for more of the same from his charges.
Nobody's effort could be faulted and skipper Joss Labadie, in particular, did not stop running in midfield – while also managing to keep his emotions in check.
Vale, however, made two changes for the start of the second period.
Clarke – who was not afraid to take such measures during his time at the Banks's Stadium – brought on ex-Walsall defender Mal Benning and striker Devante Rodney.
They looked better for those tweaks, too, with Rodney's strength causing a couple of issues.
The Saddlers made a sub of their own in the 54th minute as scorer Kiernan limped off, Shade coming on, while they looked to keep it tight for a spell.
Monthe carried on doing his bit in that regard, stopping Amoo in his tracks again.
Tensions were rising, and for Vale, they ended up spilling over.
A rash challenge from James Gibbons on Ward sparked a series of yellows – home captain Conlon among those to go into the book.
And before long, Walsall had a man advantage as Conlon caught Earing late for another caution and an early shower.
Taylor had the difficult decision of whether to stick or twist for the final quarter of an hour, but in the end, they played their cards right with a mixture of both.
A superb night for the Saddlers. This win, without doubt, will taste so sweet.
Vale (3-5-2): Covolan; Cass, Smith, Jones (Benning, 46); Worrall (Rodney, 46), Garrity, Pett (Proctor, 69), Conlon (c), Gibbons; Politic, Amoo
Subs not used: Stone (gk), Walker, Legge, Martin
Red card: Conlon (73)
Walsall (4-2-3-1): Rushworth; White, Menayese, Monthe, Ward; Labadie, Earing; Wilkinson (Kinsella, 71), Osadebe (Phillips, 80), Kiernan (Shade, 54); Miller
Subs not used: Rose (gk), Taylor, Perry, Khan
Goal: Kiernan (29)
Attendance: 6,248 (1,564 Walsall fans)
Referee: Simon Mather