The Somerset side booked their place in the southern area semi-finals after first half goals from midfield duo Ed Upson and Sam Foley saw off the Chairboys, who had Matt Spring sent off after half an hour.
Johnson was delighted to book his place in the next round - but slammed his side's laboured second half performance.
He said: "It was a professional job and we started well in both halves, but the game died a death.
"Most of the lads worked hard and it looked like we fancied it at the start, but if you look at the stats I think we must have had 30 chances and I wanted to take more of those.
"You rarely get a chance for the lads who haven't been in the starting 11 to go out and be lively, strong and passionate, but it didn't happen and the game petered out.
"It would have been nice to get a third or fourth to make the fans a bit happier, but we have to be honest with ourselves and accept that we still have a bit of work to do to make us competitive.
"We're through and that's the positive, and it's nice to only be three games away from Wembley.
"We have had three games up to now and haven't been great in any of them, but we've scored more than the opposition so hopefully the draw will be kind to us and we can look forward."