"We looked at percentages after the Port Elizabeth Test and we felt we were at about 75 percent," Vaughan told a news conference on Friday.

"Obviously you want to play at 100 percent, but we felt we played at about 85 or 90 percent throughout the year.

"So we can improve, and if we can do that we'll put the South Africans under a lot of pressure."

One of England's problems during the first Test in Port Elizabeth was the inability of fast bowler Steve Harmison to find his rhythm and length.

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Harmison went into the series as the world's top-ranked fast bowler but took just one wicket for 142 runs, although Vaughan was confident the paceman would find his form soon.

"Steve started to look like his old self in spells (in Port Elizabeth), and he will be a huge threat to the South Africans in the series," Vaughan said.

"When Steve's bowling well he glides, and he's probably just trying too hard at the moment and putting in too much effort.

"There was a four-over spell in the second innings when the ball was starting to glide through to the wicketkeeper and that's good."

England are likely to stick with the side that beat South Africa by seven wickets in the first match to notch a record eighth successive Test victory.

The only doubt is fast bowler Simon Jones, who took four for 39 in the second innings at Port Elizabeth.

"Simon Jones has a slight bug, just a bit of flu, but he's had a bowl and I'm sure he'll be fine," Vaughan said.