I've only lost vaccum in a single once - I was in VMC, but it was a
very dark night that seemed to get even darker
. Picked it up when the AH started to show a gentle bank to the right while the DG started turning gently to the left. (What if they'd both started showing a 'phantom' turn in the same direction ...?? )
The one good thing about it happening at night was that this aircraft had post lights, and I could turn them away from the AH and DG so I didn't keep seeing them. It is
bloody hard to ignore an AH which looks 'normal' and is showing a gentle turn ...
Since then, I carry a packet of cardboard instrument covers that fit into the steam gauges, clipped on my knee board. Still got the price tag on 'em: $2.50 (admittedly 10 years ago - but I haven't needed to use 'em since). Well worth the investment!
Ted