VGA connection is analog, and is different from HDMI connection (digital). In my experience, you're pretty much limited to 720p and 1080p for HDMI. As mentioned, Metro requires 1024x768 minimum (and 1366x768 for snap). Ask any netbook (1024x600) owner.
The solution would be to either swap out the large display for a 1080p model, or continue using it as an extended (2nd) display.
To move apps between displays (using "extended" display setup), suggest trying Winkey+PgUp/PgDn as mentioned in this link,
There are other relevant key combos here in this complete shortcuts listing, search on "monitor"