You have all your DC connections taken to one point. You have earth connections (presumably cable screens and similar) taken to another point, and I'm guessing that this is connected to the control box metalwork, machine frame, etc, and includes the external mains earth. If these two points are ONLY connected via the VGA cable and monitor, then there isn't a loop. A loop means that current can get between any two points by two different routes. In your case, there is only one route between two points. However, what this means in terms of interference rejection (which is why you usually try to avoid ground loops) is pretty unpredictable.

Personally, I have combined DC grounds and all the screen and protective earths at one single point. But the only cables leaving my cabinet are to items on the machine (motors, etc) which are isolated, the mains cable itself with its protective earth taken to the common point, and the ethernet cable to the PC - and ethernet has ground isolation built-in, which is one of its advantages. I would be inclined to connect your two grounds together in the cabinet if only to ensure that there is a solid and reliable grounding throughout. I don't like the idea of a random "is it/isn't it connected" situation depending on a VGA cable.