The 23-year-old finalised personal terms on a four-year deal after passing a medical at Villa on Thursday.
Reo-Coker made 142 league and cup appearances for the Hammers and is looking forward to a "fresh start" after a tough season at Upton Park.
The England Under-21 skipper said: "People ask me if I felt I was hung out to dry at West Ham and I believe I was - I was made the scapegoat."
Some supporters and critics turned on Reo-Coker and blamed the club captain for the slump in form as they spent much of the campaign in the bottom three before narrowly avoiding relegation.
"It was an experience which has hardened me but I don't think it was an experience anyone should be forced to go through, like I was, at the age of 22," he added.
"But it has made me a stronger character and I will now channel that in the right way.
"If people want to see me as obnoxious or bitter about what went on then that's their prerogative but my honest opinion was that I was hung out to dry.
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"I do not know how West Ham fans will treat me when I go back to play there but whatever anybody says about me I know I have always given 110% for whichever club I've played for."
Reo-Coker is manager Martin O'Neill's first signing of the summer and is a welcome boost after midfielders Gavin McCann moved to Bolton and Steven Davis joined Fulham.
Villa had initially tested the Hammers with a £7m bid but the London club were said to be holding out for a fee nearer the £10m mark.
"I am absolutely delighted he is here and I think he is worth waiting for," said O'Neill.
"We are very pleased. He is a terrific age and is young and vibrant and raring to go, and I think he is an exceptional player who will be excellent for us."
Reo-Coker added: "The reason I joined is their aspiration to play in Europe. Martin O'Neill has told me what he aims to do and I believe we can achieve it.
"Football is my occupation and I love the game but I don't build myself up to be something I'm not. All I want to do is be successful.
"I'm not a superstar or a miracle worker. I'm a hard-working person and I give 100% in every game I play."
Reo-Coker, who handed in a transfer request to West Ham at the end of last season, cost West Ham £500,000 when he moved from Wimbledon.
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