in code behind you need to have the following code:
protected void Page_Load(object sender, EventArgs e)
{
myIframe.Attributes["src"] = "http://www.youtube.com/watch?v=NEcZmT0fiNM&list=PLp12xt0S4J0UYXerKrIPCLTk15ZUzFdKz";
}
and Also you need to make iframe to be accessible in server side by changing yours to the following one :
<iframe src="//player.vimeo.com/video/91467577?title=0&byline=0&portrait=0&color=ffffff" width="500" height="281" frameborder="0">
webkitallowfullscreen mozallowfullscreen allowfullscreen runat="server" ID="myIframe"></iframe>
if you take a look I only added these two properties to yours 'runat="server"' and ID="myIframe"
hope it helps