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Thanks very much to all for the responses. There are a lot of claims from different sources about the effectiveness of the potential mods, together with counterclaims which leave the field unresolved. At least one of the vortex device manufacturers states and continues to state that they "reduce the partial, drag causing vacuum that exists at the rear facing surfaces".
It's not for me to resolve the usefulness of the mods, other than by trying them out; that's very cheap and quick to do with a v.g.. The vehicle is anyway no fun to drive in high crosswinds. As most other additions beside a v.g. will increase the surface area from the side (and I'm not aware of claims regarding major stability improvements from such changes). There are also other benefits, so the fitting of v.g.s is for now the 'base case'.
I hope I state correctly that suitably placed v.g.s at the back of the box will delay mixing of the free/boundary layer air and the 'trapped' turbulent pocket at at the back of the box. It seems to me logical, that as there a lower pressure area there (causing the drag force), the vortex boundary should deflect inward a bit (and this is what some suppliers seem to suggest). That inward defection is what the flow defectors seem to be doing.
@aerohead: yes, I accept there is usually nothing to reattach to, though very occasionally there may be a trailer; I went through the NASA project you mentioned some while back.
@j-c-c / JSH: The vehicle chassis is a Mercedes Sprinter, so not that bad aerodynamically and the box body is the part to focus on -currently the rear is the bit to figure out; further the box should soon get removed and fitted on a newer chassis, hence no point in a focus on the old one.
I've seen the Trailer Tail, Rocket Tail, Aerofin, Airtabs, among others:
Youtube: Inflatable aerodynamic trucktail for cargo trucks
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Latest truck aerodynamics made in Germany Trucksack .
However, as overall I don't do high mileage, I'll be going for a simple solution and there is a bit less than a foot / 30cm between the end of the box and end of the vehicle, hence I could avoid overhang. Plus if it's a commercial fit it will need to have an EU type approval and I never yet saw a truck with any recognisable aftermarket deflector anywhere in Europe, though I read some are approved.
Back to the original query, if the consensus here is that v.g.s won't improve CD, then presumably any of these rear deflection ideas would yeild useful improvements in combination?
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