Darkorbit doesn't even have to utilize the GPU, thanks to flash. It used to only do rendering on the CPU and the only work that could be done by the GPU clipping, drawing and maybe shaders. Last time I checked it didn't do clipping nor shaders, so chances are it didn't change.
Check your browsers GPU usage while playing darkorbit (having hardware acceleration turned OFF in flash) and see for yourself. (I suspect the only work being done is blitting the image hence the small amount of usage.)
So hosting PixelBots on a vServer should be possible GPU-wise. Note that Darkorbit is very CPU and RAM intensive though, so you might consider using a dedicated server with desktop CPU's. Also note that darkorbit will use at least ~500 MBs of RAM no matter what quality settings you choose. Then add the overhead for the browser and rest of the system.