Hi Adam
I had my TDI on VSI serviced at the beginning of June this year when it was exactly 2 years old and having done just over 23K miles.
As of today (4 months later) the mileage is 26K and the distance to next service is almost 14K. A bit of a shock as I was expecting about 20K.
But having done 3K miles in 4 months then on the same basis I will do 15K over the next 20 months up to the second full service at the 4 year point.
So it looks like the VSI has time built into it's calculation.
I believe that the VSI takes into account the number of journies and their length, so that lots of short journies reduces the service interval. If that is true, then I'm wondering whether I shouldn't turn of the engine when waiting at some traffic lights. At some lights I know, the cycle time can be quite a few minutes, so turning off the engine saves fuel and the planet. But if that comes at the expense of more frequent services then it may cost more than the fuel saved!