First, you are correct that the gluten can absorb more water than the wheat flour.  In short; yes you need to add more water for the gluten, most likely.  Conceptually the gluten protein in the flour is the main thing absorbing water.  The starch granules get coated with some water, but don’t absorb it until they are unlocked by heat in baking.  Damaged starch granules create more surface area, so that wheat flours with higher starch damage seem to absorb more water.  By adding 8g VWG, you end up with 272g flour at 14.7% protein.  If you take 8g flour out and replace it with VWG, then you will have 264g flour still at 14.7% protein, and for the poolish I would then use the same 264g water.  Some bakers like to add the gluten to the dough side.  The fermentation can break the proteins down and loose strength.  Vital wheat gluten does need time to hydrate completely, an autolyase and or extra mixing time is recommend When adding to formulas.
I hope this helps you