I'm typing this from a phone so pardon any mistakes in spelling :p
The main difference is in strategy. Singles involves a lot of setting up a specific thing, and banking on them doing enough damage or sweeping, and / or your opponent not being able to deal with it one on one. VGC is more fast paced, in the way that you can't expect constant switches to counter something the other player has, on your part or your opponents. For this reason, you'll have to adapt to teams that focus less on entry hazards, less on switching into a pokemon that can't do anything to set up, and less on any one pokemon on a team in particular since if something gets too threatening in any doubles battle, it'll attract more attention. Oh, and yeah attracting attention. Get used to that in doubles; you have to predict more than just which slight variation of build your opponent is using on a pokemon, you have to gauge the situation to predict where they'll attack, if they even do on any given turn. Protect games make or break a lot of battles in VGC.