"We will need to wait, not for very long, for a new administration to be established and at that point we need to reveal our strategy, not only a US strategy but the overall strategy, because clearly what we are doing so far is not going to lead to success."
Mr Vendrell, a veteran Spanish diplomat, left the EU post in Afghanistan at the end of August.
"I don't leave with a sense of failure," he said.
"But I do leave with a sense of regret that we made so many mistakes. I don't believe the situation will lead to failure but we have got to do a hell of a lot to get things right."
Mr Vendrell added his voice to the criticism of civilian deaths in Afghanistan from aerial bombings. "It is doing us an enormous amount of harm with the public," he warned.
"In 2002, we were being welcomed almost as liberators by the Afghans. Now we are being seen as a necessary evil, perhaps something that they need to put up with because our departure would probably mean a civil war, but these kinds of actions completely undermine the efforts to win hearts and minds."
Mr Vendrell's successor as EU envoy to Afghanistan is the Italian diplomat, Ettore Francesco Sequi.
HARDtalk is broadcast on BBC World News at 03:30 GMT, 08:30 GMT, 14:30 GMT, 20:30 GMT and 22:30 GMT.
HARDtalk can also be seen on BBC News at 04:30 BST & 23:30 BST.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.