I think we can say the vanos is not preventing the valves closing . Carbon deposits are more likely to cause that .
Problem sounds electrical to me . Or could be many things . There are knock sensors on the heads under the intake manifold which can cause problems . Not saying it`s these but just maybe a possibility .
The vanos could be the cause as you have discovered but I think you really need a definitive answer .
There are 2 BMW techs on this forum could give you a good idea of what to check . They are not around much these days . Weasel and Killcrap are their usernames .

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