Steam Workshop support is now in the game, in a VERY early Beta stage. Though I believe it works ok right now, I have no idea how it'll handle lots of people uploading vehicles. I will be monitoring feedback closely and will make necessary updates ASAP. That may include disabling it for a short while if there are major problems.
Please respond to this update or post in the forums with issues. I'm not really concerned about feature requests yet, as I want to get the basics working, then I'll move onto things like upvoting, updating your own vehicles, and making the sorting/tagging options work well.
As for how it works, I think it's pretty straightforward, so I'm not going to explain anything but the essentials here. You can upload vehicles, and you can browse vehicles. When you browse, you have to click on the vehicle once to download it. When it's downloaded, it gets a checkmark, at which point you click it again to load it.
I know some of the UI (particularly text input stuff) is wonky.