This free tool will create a valid XHTML embed code for any Vimeo video. The code Vimeo shows on the embed field is not valid XHTML! However, you can simply use this simple tool to make it Valid XHTML 1.0 Transitional.
Tip: There is also a similar tool to generate valid XHTML Youtube embed code, as well as one for valid XHTML MySpace embed code.
Vimeo embed tool
Validate your code!
Remember that you should always validate your code to ensure maximum compatibility between browsers.