A friend of mine has a vivaro, and he had a system fitted 650L by ionics pure2o, they said they couldnt fit it in the middle and fitted it straight over the rear wheels.
If that was me i would have made them move it, 1 they were just being lazy, and 2 it has probaly overloaded the rear axle weight limit.
my ionics tank was fitted in my previous vehicle over the rear wheels - but that was at my request in a 1200 kg payload t4 transporter so was not over the axle limit , this van has a very heavy 2.5 engine at the front too so handling was fine.
My grippa install in my vivaro is mid mounted and they said that was the best place to fit it for equal weight loading and due to the position of the fuel tank .
They both use a different way of mounting , grippa use big bonded plates through the floor plan , ionics bolt through chassis , maybe its the position of the chassis rails or parts blocking the rails that prevented them fitting it further forward .
if its over the axle limit then they shouldn't have fitted it in my opinion .