Cabrera and Vegas hold a one-shot lead at the Robert Trent Jones Golf Trail at the PGA Tour event.
However, the top of the leaderboard is heavily congested.
Martin Laird, Shawn Stefani, Blayne Barber, Matt Bettencourt, Michael Kim, Ben Crane and Lucas Lee are all tied for third.
Argentinian Cabrera was in fine form in Opelika, Alabama as he mixed four birdies with an eagle – the latter coming at the par-five 13th.
Similarly good was Vegas, who went through bogey-free and also had an eagle at the 13th.
Among 10 players at four under are Australians Stuart Appleby and Jarrod Lyle.