There would have been a quartet at the top of the leaderboard but for the dramatic disqualification of Hennie Otto, who moved the 150m marker to play his approach shot at the fourth, not realising it was an immovable obstruction, the removal of which constitutes a two-shot penalty, and hence signing for a wrong score before the error was pointed out, which resulted in his exclusion.
Vaughan, who shot 65 and 68 at Sun City for the halfway lead in last weekend's Dimension Data Pro-Am, reversed those scores when he completed a seven-under-par 65, including six birdies, an eagle and a single dropped shot, to total 133 after 36 holes for the second week running.
This time he shares the lead with 1985 champion Williams and Gammon, who were among the later finishers and will play together for the third day in a row on Saturday, joining the American in the lead three-ball.
'The last two days, I've hit it as well as I've done for a while,' said Vaughan, who said he'd got a swing tip - `to stick my ass out'- the other day from fellow touring pro and close friend Derek Crawford, the Scotsman who was only three behind him after a 69 on Friday morning.
Vaughan said he was struggling to read the greens on the new Woodhill course, although he did manage to hole two monster putts, from around 40 feet, for birdies on the 14th and 18th holes. 'I made a couple of bombs, but I missed several along the way, but if you shoot 65 and you're complaining, something's wrong,' said Vaughan.
'Our games carry each other and we both need to do well,' Williams said of his and Gammon's identical scores thus far. 'The pressure of this course is that you have to make a lot of birdies in every round to keep in touch.' Williams said he didn't think the winning score would be as low as 20 under par, but said it was possible if conditions stayed relatively windless.
Gammon started his round with what he described as 'the worst tee-shot of my life' on the 10th hole, but followed it with a brilliant second to three feet. Although he missed the birdie, salvaging the par was a boost to his confidence and he went on to make three birdies on each nine and just a single bogey. He said he still expected someone to shoot a really low round on the new lay-out. 'There are no holes that are real back-breakers out there,' he said.
Overnight leader Ulrich van den Berg, who followed his opening 65 with a 69, was one off the pace while De Wet Basson, who won the Fancourt Hall of Fame title way back in 1992, but has struggled in recent times, shot a bogey-free 66 to be on 135, two behind the leaders and level with Deane Pappas, who shot a 67.
The cut was made at one-over-par 145, with 78 players making it through to the last two rounds.