I believe I have one of these device (mine is branded as something else but the US brand is the ViewSonic gTablet). I cannot recall if I have tried to do a screenshot with DDMS or not. I don't know why the forums you read would say adb would work becuase ADB uses the same method as DDMS.
I will try my device when I get home from work today, in the mean time, I have read that these apps should work with the device.
All of these are paid apps, they may have a free/lite version but I dont know if they will work for your issue.
Screenshot ER - Requires Root
Screen Shot It - Requires Root
No Root Screen Shot It - NO ROOT REQUIRED (but I am not sure if this works)