The first step with such frameworks is usually: "learning". That is: you have to show vehicles of known type to the framework, and tell it the type. Only after that step will the framework be able to classify.
Best approach is to start with simple tasks, like simple pattern detection, and thus get some knowledge on how it works before you start such complex tasks. There are many articles on AForge here on codeproject, see
https://www.google.com/#newwindow=1&q=site:codeproject.com+aforge&safe=off[
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