Greg,

I don't know the statistical probabilities but it is clear that they can and do fail and the result can be serious damage to the engine. Apparently, some of the internal parts of the pump can break off and get down in the engine, causing bad things to happen. Also, a vacuum pump failure can lead to oil getting into the car's various vacuum lines which must then be cleaned out at considerable effort/expense.

The Mercedes Club of America has a "maintenance" frenquency discussion that suggests replacement by 150k, but Larry notes in his comments that they make a certain sound when they are starting to break, so he is comfortable waiting for that indicator.

All 123 series (617 moters) have this issue and, I suppose, some of the other vacuum oriented diesel motors.

There are repair kits available as well, but I don't know how reliable they are. Probably depends on skill of re-builder.

Nic