An 870? I mean, the min was a Nvidia 960 when it came out, and then later changed to 970. Sooo.. you are a little behind.
How will it run? Umm.. if it runs, it might give you motion sickness. The real question is will the Vive just not even let you install / run without meeting the min specs. No idea!!!
Even if you get it to run, is all you want to play is Beat Saber (and poorly).
I think the real question you want to ask is, "I have 6 hours left on Black Friday, where are the best deals on Graphics Cards that are VR ready?"