FFF
Partial panel work that you do for the IMC rating is not enough, IMHO, because there is no requirement to recognise the failure
I don't believe this, because even the most knackered just-about-IMC-capable spamcan will have an AI (vac) and a TC (electric). The pilot is taught to scan both of these, and if flying something this basic he ought to be aware that he must really watch things.
Anything less should not be flown in IMC at all, because a vac AI fails gradually and it isn't at all obvious until you are upside down.
In any plane which someone might be flying in IMC for real, there should be a vac warning light which should be obvious when it lights up. There will also be a second (electric) AI which you also keep an eye on.
I suppose that incidents where vac failures cause accidents are where the pilot has such a high workload that his scan goes down the pan (easily done if there is no autopilot for example) or where he is flying a plane which isn't up to the job. None of the planes in which I trained I would fly in IMC myself - the workload is very high. Even a non-slaved DI which drifts 10 degrees every 5 mins (pretty normal for a spamcan) increases workload drastically.