Media FX should apply only to input clip but it does not allow to extend duration. It’s Video Track FX that applies to whole video track.
Do you have in and out point set in your clip, if not it should just work (you do like in our tutorial and doc)?
There is another workaround which is to load your clip in Vegas by itself, save it as .veg (you can then sub-clip it, like set initial frame start by sliding it) and import that into your main Vegas project as if it was a clip.
Pierre