I have only been flying with VS since April but have had several op ups to UC since then. My feeling is they will not do it just because they have an empty seat (unless, it seems, you are VS staff but I wont get into that !).

They will only move you one cabin up and only if it is oversold in your booked (ie. paid for) cabin.

Generally it is decided on FF status AND booking class. If they need to move one person and have two Gold members in the cabin my view is it will then go by who paid most for the ticket (that is fair) but if both Golds have the same ticket type then would be decided by who has the most tier points.

I don"t think there is anything random about it but it looks random.
Of course just to hedge your bets a bit of flirting with the check in clerk wont hurt (male or female - who cares - we are talking nicer seat here !!!)