I'd go with another Q in the back.. That's what they were designed for, to wrap that sound all around ya.. The VP speakers are great, but in my humble opinon not really at their best as rears.. Too easy to identify the speakers location during movies, if you know what I mean ?
Make it a QS8, since it would be running solo.. ? Just a thought, probably not necessary..

LFE ! The rest is just details..