Had a Vito in a while back with a similar problem, faults on both rear sensors.
I just didn't believe both sensors could be at fault, so dug a bit deeper, the van had been fitted with a second row of seats, crew cab style, which I assume was installed after manufacturer, looked a professional job though, floor boarded and the trim all matched.

Anyway I came to the conclusion the loom was squashed underneath, rather than pull out all the interior, I added two new wiring looms front to rear under the van and problem sorted.

Chris