Maybe it's grouped too deep. I had a few pieces of render missing when I grouped a group inside a group inside a group etc. Try also resetting to default vray render settings, if you haven't tried it already. What also might be the case is that you have some material ID channel, or some other channel, chosen in Vray frame buffer. In upper left corner, check that. What's strange here is that those chairs on top don't look like the chairs in bottom picture...