D800 with 24-70mm and VR question.

"Onion" bokeh is pretty common with zoom lenses, including Nikon's own 24-120.  It isn't something you will see all the time...most noticeable with round bright highlights...like out of focus small lights.  The Nikon 24-70 is very good in this regard.  If this is something that will bother you, the Nikon is the way to go, but realize it exists for lots of lenses...not just the Tamron.