Why am I not able to put videos onto my iPod Classic?

I have an 80GB iPod Classic, which has tons of space left in it, and I understand that MP4 files can be uploaded to it. I am using Windows Vista, and there are two videos from Vimeo that I have downloaded (in .flv form) and converted (into .mp4 form thanks to zamzar.com), yet when I am in the process of syncing the device I get the following message for both videos: ”[Video Title] was not copied because the video format is not supported by the iPod”. Hopefully someone out there can help me out as to what I need to do to get the videos onto my device. They are both under two-minutes in length, incase that helps anything.