how can i convert a skp model/material into mdl/vtf file?
Well you aren’t going to be able to do it direct from SketchUp, it’s just not used for this. But you can use a converter to make it a .3ds, .Obj, and many others. Heres a quick google search
You can also download an 8 hour trial where you can load models dirrect into it if its a certain file type, and then export it as an OBJ or 3ds format. I don’t know how reliable this way is when dealing with the textures, but it should work in theory.
and what can i do with the valve sketchup plugin?