That's not C#. It's Javascript. I doubt that the problem is with Mono, I suspect the problem is with the browser.
Since the iframe is created by Ext.Net you have to reference it with the name Ext.Net gives to it. To get the name just use the developer tools of the browser, but it should be pnlAjax_IFrame
.
Or, if it's the only iframe in the parent page you'd better reference it by id:
frmAjax = parent.frames[0].LoadVehStore2();