I rarely hear of vacuum lock failure with the W124 cars. They use essentially the same system as the W126 and W201 cars - the vacuum / pressure is provided by an electric pump which rarely fails. Also, only one vacuum line needs to run from each actuator to the pump.
However, W123 and previous models used a vacuum reservoir and required that there be two lines run to each actuator, doubling the potential for leaks.
The current C-Class and new E-Class no longer have vacuum operated door locks. They are electric just like every American and Japanese car out there. As a matter of fact, the vacuum locks are one of these little things that I LOVE about MB's. It's something different and I just find it cool. The vacuum locks on both my W124's operates flawlessly. I admit that the use of the electric vacuum pump is superior to the older design with the reservoir though.