you do not need ImgBurn for burning with dvdfab. dvdfab gold and dvdfab platinum come with VSO burn engine installed.
you can however choose to use ImgBurn or Ner0 for burning from dvdfab settings. you do need to have them pre-installed.
i recommend installing and using ImgBurn with dvdfab and not the included VSO burn engine. once you get used to using ImgBurn there are many other great features besided burning that ImgBurn can perform.
here are a couple tutorials i made which may help.