I need an external camera to be able to detect some markers positions and unity to place GameObjects at these positions.
Does anybody have any idea if there’s such a plugin or an augmented reality library which I can use in Unity to create PC games ?
As far as I know, Vuforia only allows us to create apps for iOS and Androids.
Regards, sRodelb
you should go to the AR component, open vuforia configuration under vuforia behaviour script, then look for Webcam.Find camera device and there you can switch from your computer cam to the usb one that you have already plugged in your computer. 
Piece of cake
Hi srodelb,
Really the question sound is nice.
Even I am also eager to know the answer for this.If anybody knows help for us thanks a lot.
You can use ARPA Plugin Unity to create PC Games with Augmented Reality.