The sensor detects if a person is present at the passenger seat and/or if a child seat is present. The passenger airbag is deployed accordingly.
I don't know about a Vito but on sedans the wiring/connector may fail in addition to the seat sensor. If it isn't the connector/wiring, you have to install a new sensor (it can be version coded off but I would not do it).
The first is standard fitments, from around 2004 onwards (depending on the car's model). It has two purposes. The first is to prevent the passenger air-bag from deploying when the front seat is unoccupied and thus reduce the repair costs in case of a collision (as requested by insurance companies who were reluctant to pay-out for two new airbags when the front seat is un-occupied).
The second is that according to Mercedes Benz they have a central controller which allows for differential deployment of the passenger air-bag based on the person's weight, so that children or slightly-built adults are not hit with the full force of the inflating air-bag.
The sensor is in the shape of a wide and thin wide mat with several flat switched located all around and a controller box, fitted inside the seat cushion. The array of flat switches allow it to detect the person's weight. If there is no passenger in the front seat, the air-bag will not deploy.
This often fails and needs replacing. Olly replaced mine, the cost for the C-Class was around 135.00+VAT for the part plus 1-2 hours labour for removing the seat and separating the cushion (apparently it is more tricky to do with leather seats than cloth seats).
When this fails, you will get the 'SRS visit workshop' message and the SRS light will come on (probably only intermittently at first). I don't know for fact if it means that the entire SRS system and all air-bags are deactivated, or just the passenger air-bag.
I have an MB PDF document from 2004 stating quite clearly that when replacing the Front Passenger Seat Occupancy Sensor the SRS system needs to be coded. But Olly said this was not the case and successfully replaced mine without any problems.
ASKE, on the other hand, is an optional feature which, when used with MB ISOFIX child seats, deactivates the front passenger air-bag while the seat is connected.
For this purpose, there is a light on the dash that comes on when the front air-bag is deactivated. While the light itself is fitted to all cars, not only those with the ASKE option, on cars without ASKE is has no purpose whatsoever (practically for decoration only..).
Now here's the tricky bit... strangely my car's Datacard states 'ASKE' when I know for fact that I do NOT have this option (as confirmed by the sticker on the side of the dash, visible when the front passenger door is open), and I therefore believe that MB actually meant to say 'Front Passenger Seat Occupancy Sensor' rather than ASKE. I have also seen this 'error' on other members' cars when checking their Datacards for them. So in short the fact that your car's Datacard says 'ASKE' does not necessarily means that you actually have the 'Automatic Child Seat Recognition' system.
Incidentally, on my car the two wiring connectors (seat controls and SRS sensor) are at the base of the seat, on its frame, at the front side - one yellow one blue.
My car is 2000 , it has just the akse. MB tell me the sensor is faulty. If i unplug the sensor under the seat then the dash indicator lights up meaning 'Airbag off' . Replacement quote from MB is 560 euro. Sensor costing four hundred and something alone. I'd sooner try repair it or something. My car doesn't have a datacard. Imagine you buy vito van then stick seats into and windows. Thats about the level of equipment they put into the v-class ! WIS shows a symbol of a transistor in the diagram but that could mean anything. Maybe a removal is the only way to see what it is. If it's removed from the program using HHT will the abs fault light go out ?