[NTG-context] fonts in context (general question)

hello i have a general question to the use of fonts in context. is that true that i always need a *.tfm and *.pfb file of a font to use it in context?? if i generate some *.tfm (from afm files) with "texfont" context complains about not finding the *.pfb files. when I use texfont *.tfm and *.vf are generated. what are the *.vf files good for?? thanks for an answer. greets severin