Setup guide
A Valve Steam Controller can drive in Vsimulator — but not on its own. Unlike an Xbox or PlayStation gamepad, the Steam Controller only makes sense through Steam Input, which translates it into a normal Xbox controller that your browser understands. Once that's on, the simulator sees a standard Xbox pad and it works with no extra setup on our end.
Run Steam, turn on Steam Input for the desktop (or add your browser as a Non-Steam game), and pick the Gamepad layout. Vsimulator will then detect an “Xbox 360 Controller” and you can drive. Without Steam running, the controller behaves as a mouse and keyboard and the simulator can't use it.
The Steam Controller has two trackpads, one thumbstick, and gyro instead of the usual two sticks. It isn't a “standard” gamepad on its own, so a browser can't map it sensibly. Steam does that translation for it — presenting a virtual Xbox controller to everything else on your computer, including Vsimulator. This applies to the original 2015 Steam Controller (the discontinued one) as well as newer Valve controllers.
Steam isn't running, or Steam Input isn't enabled for the desktop/your browser. Without Steam the Steam Controller runs in its built-in “desktop” mode — the right trackpad is a mouse and some buttons are keyboard keys — so it never appears as a gamepad. Start Steam and enable Steam Input, then reload.
That means the raw controller is reaching the browser without Steam's Xbox emulation. Enable Steam Input and pick the Gamepad template so Steam presents it as an Xbox pad. (If you'd rather use it as-is, you can still map the sticks by hand in the calibration section of the controller settings.)
Your Steam layout is sending the trackpads to the mouse instead of a stick. Switch that layout to the Gamepad template, or bind the left trackpad/stick to “Left Joystick”.
For a deep dive on configuring the Steam Controller itself, the community Steam Controller 101 guide is a good reference. For the best plug-and-play experience in Vsimulator, an Xbox or PlayStation controller needs none of this setup.