As usual, internet search threw up several possibilities - typically that this indicated failing duckbill valves. Also suggested that this issue was rarely associated with the vacuum switch.

However, initial investigation showed that there was nothing wrong with the vacuum; returning to the boat after 48 hours, the toilet vacuum-flushed strongly, even though I had left the domestic power off for two days before testing.

To access the two vacuum generators, it was necessary to pull up the carpet in the master cabin, because for some reason the previous owner had laid the carpet directly over the access hatch, rather than fitting the carpet to it. I don't know whether this is standard on the Phantom 46.

I also located all the documentation for the vacuum generator, and discovered that one unit had been replaced. The port ensuite toilet had a VG2 model, but the starboard guest ensuite/day head toilet had a newer VG4 one.

Interestingly, the parts list for the VG2 unit had the vacuum switch part number asterisked.

Removing the top cover provides access to the vacuum switch.

The first thing I noticed was that there was a gap between the brass plunger and the end of the limiter switch (white nylon protrusion).

The switch was stuck in the on position, but could be turned off by assisting it with a penknife.

I was unable to rectify this in-situ, so I took the switch out and took it home for cleaning. It looks difficult to disassemble, but I have a suspicion that someone already tried this because one terminal is missing, along with the corner of the switch housing:

 

As you can see from the photograph above, one wire is soldered into place and the terminal post is missing.

This might explain why there was an asterisk next to the part number for the whole vacuum switch assembly!

I have located a couple of places that can supply the whole vacuum switch assembly, but before I splashed out on this, I discovered that the limiter switch itself (9007A01) is easily obtainable from RS or Farnell.

I've ordered one of these, but for now, I managed to get this existing switch to work again using some 3-IN-ONE Silicone Lubricant. I find this useful for any applications that require dry lubrication; light oils can be prone to gumming up or can get into the switch itself. Whereas this silicone spray evaporates leaving behind a very fine film.

This worked very well and the toilet is functioning properly again. I don't know how long it will work for, but the replacement switch is on the way.