JF,
It was air-conditioning smoke. The VC10 did not use bleed air but had mechanically driven blowers on each engine. A gear box seal in the number 4 blower failed dumping all the gearbox oil into the compressor part of the blower where it was instantly vaporised. Fortunately the smoke, though very thick, was non-toxic and non-irritant otherwise we wouldn't have lived to tell the tale.
The flight engineer managed by feel to dump the cockpit air supply (see:-
IFR conditions on the flight deck
It is interesting after such an event to compare memories. No-one really knows the overview and each of us had peculiar snapshots of bits of what happened.
What I do know is that that incident made me obsessive ever after about trying to know every facet of the aircraft I was flying.