Lancashire 148 for 4 (Vilas 44*) beat Yorkshire 145 for 9 (Wood 3-21, Parkinson 3-25) by six wickets
Lancashire took advantage of Yorkshire's lack of four senior players due to a coronavirus-related issue to seal a commanding six-wicket Vitality Blast win at Headingley, taking a significant step towards the quarter-finals.
The Vikings were without Matthew Fisher, Tom Kohler-Cadmore, Josh Poysden and captain David Willey, and they now have a mountain to climb to qualify for the last eight. The club released a brief statement, saying that the players had "been withdrawn from tonight's squad in accordance with Covid guidance" and adding that results of a Covid test would determine their future availability.
Yorkshire actually made an excellent start, reaching 62 for 1 after six overs having elected to bat before stumbling to 145 for 9 after 20 overs, with Matt Parkinson and Luke Wood taking three cheap wickets apiece.
The Lightning, far from their best in the field and with the bat, then chased down the target in nervy fashion at times. They recovered from 77 for 4 in the ninth over as captain Dane Vilas top-scored with 44 not out off 36 balls, winning with 13 balls to spare.
They have consolidated second spot in the North Group with a fourth win, moving to 10 points - two behind leaders Nottinghamshire. More importantly, they are four points ahead of Yorkshire on six points in fourth.
Both teams have three games remaining, including Thursday's return clash at Emirates Old Trafford. The top two teams from the three groups qualify for the quarters, as do the two best third-placed teams.
After a racing start, Yorkshire's innings fell away through the middle, with legspinner Parkinson striking twice in his first three balls to remove Dawid Malan and former England Under-19s allrounder George Hill.
The Vikings slipped from 78 for 1 in the ninth over to 87 for 4 in the 11th, with stand-in captain Adam Lyth also falling for an innings-high 36 having been dropped twice.
With no Willey and Kohler-Cadmore, Yorkshire's batting order was particularly inexperienced, with academy batsman James Wharton, aged 19, making his first-team debut.
Lyth and new opening partner Harry Brook took 15 off each of the first two overs, both pulling huge sixes into the Western Terrace.
Brook was the first wicket to go in the third, brilliantly caught by a diving Tom Hartley running around from short third man off Wood - 30 for 1.
Malan, a reserve player in England's bubble for the ODI series against Australia, was released to make his Yorkshire T20 debut and played confidently as the hosts reached 62 for 1 after six overs.
However, after Lyth was stumped off Hartley's left-arm spin, Malan was the first of the two wickets to fall in Parkinson's first over when he was caught at deep mid-wicket on the slog sweep. Two balls later, Hill chipped to cover.
Wickets continued to fall in excellent batting conditions as spin dragged things back for a somewhat lucky Lightning side who dropped a trio of catches - two from Tom Bailey.
Wharton, like Hill, was caught at cover off Parkinson - 104 for 5 in the 15th - before Mat Pillans was stumped off Liam Livingstone's legspin almost two overs later as the score fell to 121 for 6.
Left-arm seamer Wood then snapped a run of five wickets to spin when he struck twice in the 17th over, getting Jordan Thompson lbw and Will Fraine caught behind for 24 - 124 for 8. Danny Lamb got Ben Coad in the last over of the innings.
Coad then conceded six wides with the first two balls of the Lancashire reply before Pillans and Thompson removed openers Livingstone and Davies. Livingstone was caught behind and Davies holed out to deep square leg as the score fell to 35 for 2 in the fourth.
Lancashire maintained a healthy run-rate, only to lose Steven Croft, run out following a mix-up with Vilas, and Josh Bohannon, bowled via inside-edge by Coad, in successive overs - 77 for 4 in the ninth.
But Vilas and Rob Jones steadied the ship with Lancashire's best fifth-wicket partnership in the history of this fixture, an unbeaten 71 inside 10 overs.
Jones hit a six over long-on in a career best 38 off 35 balls and hit the winning runs to ensure the Lightning are now unbeaten in this fixture since August 2017.