Ask at check in about using your miles to upgrade. They can only say no but equally they may be happy to take some miles off you.

But VS (like BA) don't just upgrade elites for free because there is space available in the cabin. I've been on flights with plenty of space in UC but heard staff at checkin and at the gate state there were no upgrades 'of any type' available.

They do it when it suits them e.g. As mentioned when they have over booked a cabin and need to shuffle people about.