Johnson was Vale's hero on Tuesday night as he saved Walsall's final sudden-death spot-kick from counterpart Karl Darlow before then scoring himself to win the shoot-out 6-5, sealing the visitors' place in the northern section quarter-finals.
It capped an impressive turnaround from the League Two high-fliers, who battled back from two goals down to level the match at 2-2 after 90 minutes.
Johnson, a second-year professional who started in place of the injured Chris Neale, said: "I was a bit nervous at the start of the game, but then the adrenaline took over.
"Even when we went two down in normal time, I didn't think we would lose because we had been making plenty of chances. We'd had a joke about it before the game and I said I would score the winner from a penalty.
"Walsall's goalkeeper had made some great saves during the game so I was pleased to beat him. I just decided which corner of the goal I was going to hit it and it went in."