Overnight leader Vegas had been on course to enjoy a substantial halfway lead when he came to the last in his second round with a four-shot advantage over the field.
But that was soon halved after he made a mess of a bunker shot at 18 and then missed a tiddler of a putt to finish with a second-round 70 to go with his brilliant opening seven-under 64.
His lacklustre finish gave fresh hope to all his pursuers, including Australians Cam Davis and Adam Scott, who currently sit at two under through 36 holes.
Davis, five under after his first round, had a disappointing follow-up, slumping to a three-over 74, while veteran Scott, who'd started impressively from the 10th hole, also had a double bogey at 18 that halted his momentum.
Vegas held the clubhouse lead at eight under, two shots ahead of Frenchman Matthieu Pavon (65) and three clear of Max Homa, who had the morning's best round of 64 with the later starters still to finish.
"It was a great day," said Vegas, who before this week had never held the lead or co-lead in a major championship and was determined not to be put off by his anti-climactic finish.
"I can't really get to down on myself for making a double on 18, even though I wish I didn't make that.
"I just have to remember the good stuff and keep it up for two more days."
World No.1 Scottie Scheffler, whose opening 69 was the lowest score from his high-profile group, Masters champ Rory McIlroy (74) and defending champion Xander Schauffele (72) were all among the late starters.
US Open winner Bryson DeChambeau had a three-under-par 68 but capped his day with a disappointing bogey that left him five shots back of Vegas.
For Australia's former British Open champion Cameron Smith, though, there was no way back after his opening-day 78 as his level-par second round ensured he would miss the cut, which was set to be at one over.
That left Elvis Smylie, on his US major championship debut, in some peril after dropping four shots in the last five holes, including a sour double at the 18th, in a deflating finish after what had been a fine second round.
Smylie's 73 left him at one under for the tournament and waiting anxiously to see the cut mark.
Min Woo Lee, who was three over through 12 holes, and former winner Jason Day (four over after 14 holes) both had big ground to make up, while Karl Vilips (six over after 11) looked certain to have a weekend off.
Other notables in the clubhouse set to miss the cut were grand slam-chasing Jordan Spieth (two over), two-time winner Justin Thomas (three over), Hideki Matsuyama (three over), Ludvig Abverg (three over) and Masters runner-up Justin Rose (nine over).
With agencies
