Vaughan, who made the World Cup after a year out with knee trouble, has failed to find form and his technique was exposed by Australian paceman Shaun Tait in Sunday's seven-wicket defeat in the Super Eights.
"He's putting himself under pressure when he's had a few bad knocks and maybe the captaincy is getting to him," Fletcher told reporters.
"Having chatted to him, he looks very confident and is still full of belief about his captaincy, but he probably just puts pressure on himself knowing he has to go out there and get runs."
England must win their three remaining second stage games against Bangladesh, South Africa and West Indies to maintain a good chance of reaching the semi-finals.
England's top order have struggled to build a platform so far in the tournament but Fletcher said he was not overly worried by Vaughan's form, despite the skipper never having scored a one-day international century in 83 matches.
"I believe he is a good one-day batsman but he has got to get back to realising his true potential," said Fletcher.
"It's not as if he's batting badly. He's batting very well in the nets and he looks and feels at the top of his game."
