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Once I ran power and ground for sensor off the RoboRio, everything worked fine. This was a floating reference issue.
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This does not prove that there is a floating reference issue.
As I said earlier, the way to test that is by reproducing the problem with the VRM, then manually commonizing the Rio and VRM ground to see if it fixes the symptom. If adding a common ground wire between the VRM and RIO fixes the symptom, then it could be a ground offset. In which case remove the commonizing ground wire and measure the voltage difference to see how much the offset is.
Since neither the sensor or the VRM is drawing considerable current, AND since they are both powered by the same battery, AND since the VRM is not isolated, I really doubt there is a ground reference offset.
The problem is likely something else, something different about the two supplies or they way they are wired. What sensor are you using? What power rail are you using (12V or 5V)? What are the voltage requirements of your sensor? How much current does your sensor draw?
I'm glad the sensor is working now, but I'm a little worried later in the season it will "suddenly" start giving you trouble again because the root-cause was never determined.