Yes, the VDO sensor has a resistance output (it's a variable resistor) of about 200 ohms, but I forgot whether 200 ohms is the highest or lowest resistance.  I think it goes from about 200 ohms to zero just like old fuel level sensors in cars.  The current through it is fairly small and it's not sensitive to EMI (Electromagnetic Interference) being a simple resistor so the only requirement is to provide a "good" ground.  The most direct way would be to run a dedicated wire directly from the sensor to the engine monitor ground point at the box.  Running the ground wire to the common ground point should, in theory, be just as good, but then you could be sensitive to other grounding problems.  As long as the ground is low in resistance compared to 200 ohms you should be in good shape.  How do you attach the ground to the sensor?  A tin-plated wire laid under a hose clamp is exceedingly inelegant, but would work.  You could solder a wire to the housing and as long as you stress-relieve it by clamping the wire under the Adel clamp that would also work.  The original intent by VDO was to use the pipe threads for the ground (contrary to other information, putting thread sealant on pipe threads will not interfere with the electrical path), but if you remote-mount it with a flex line to the engine you won't get a good ground, even if the hose is metal-wrapped as there are too many anodized aluminum fittings along the way that could interfere with the ground path.  I think you need a separate ground wire.  But each solution, according to Murphy's law, creates another problem.  With a separate ground wire and even a poor ground through the hose you are set up for a ground loop condition.  The ground through the hose to the engine should be broken and the easiest way is probably to use a fabric-reinforced hose - and that would probably be the only hose on engine with a limited lifespan.  Or you could run the ground wire from the sensor directly to the engine block and that would eliminate the ground loop issue and the ground would almost certainly be adequate.