I havnt used monsters code before but it may be because you are simply forgetting to include a Header file that Vehicle has been defined in. Take another look through his code and "Vehicle" may be defined outside of OgreOdeVehicle.h somewhere. Just taking a stab though.
Are you using the "Preview" version or the CVS snapshot?
It's more likely that the CVS snapshot will work with VC6 since I think I've included some fixes that have been suggested in this area.
What does line 159 of your version of OgreOdeVehicle.h say? The CVS version makes no mention of Vehicle;
I think there's already a VC6 Version of OgreOde and the examples around. I slightly remember about downloading an E-Mail where it was attached, but as I said I'm not sure.
Going to shake google again and see if i can find that vc6 version. Will be getting vc7 as soon as IRS sends me my refund so it will moot point soon enough.