Oil in the key switch doesn't come from a faulty vacuum pump, it comes from a faulty shutoff device. The diaphram rips and the vacuum system sucks in oil from the injection pump. If the line from the shutoff to the key switch is clear, then it happened sometime in the past and the shutoff device was replaced.
The problem is that the oil ruins the rubber connectors and rubber parts inside the key switch. If you're lucky, it's just a connector. If not, then you'll need to replace that portion of the key switch.
BTW, your door locks are run from a separate vacuum pump (electric) in the trunk.
99 E300 TurboDiesel
82 300D Euro
05 Jeep Liberty CRD
00 VW Jetta TDI
Former MB: 86 190E 2.3, 87 300E, 80 240D
Chain Elongation References