Hi Rob,
The general practice is for the grade to start without the VFX shots, the look of the film is established and then as VFX shots arrive they are incorporated into the timeline so that the colourist can blend them in to the already established look, it’s a patchwork quilt of an operation. This does vary from project to project depending on many factors such as the length of the film, budget and time constraints and the supply of VFX shots from different sources.
Hope this helps,
Martin.