Hi All,
Just to run through my troubleshooting:
1) Global config allows iframes
2) Iframes allowed in JCE Editor
3) Successfully posted a video elsewhere on the site
Conclusion - VM2 is stripping the tags on its own.
I have looked in the file structure for class.inputfilter.php which is what seems to be posted online as the source of my woe... but it doesn't exist. Any help is greatly appreciated!
You should try putting the iframe code within a plugin - eg Soucerer (http://extensions.joomla.org/extensions/core-enhancements/coding-a-scripts-integration/custom-code-in-content/5051) (Plugins must be enabled in Configuratiion/Shop)
Worked like a charm, thanks jenkinhill!