Well to be honest it is a bit - no, actually it is totally stupid of VOSA to omit the chassis number from a test certificate on a vehicle that has no registration number.
One can imagine (and sympathise with) the DVLA people getting a bit aggrieved about having to refer back to VOSA every time this arises.
It is sadly indicative of our modern bureaucracy that these circumstances - which require someone to 'go the extra mile' to make things happen - often result in a stalemate, because (understandably I guess, in our era of 'taking the blame') nobody wants to go the extra mile or step outside their comfort zone (the accountability thing?). And all for want of departments talking to each other and making things simple for each other.
The same thing arises with the new 'proof of paying import duty' where HMC+E and DVLA are locked in a situation where neither department seems able to communicate easily with each other in order to help applicants through the process.