Having VR is liken to having a tripod or monopod built into the lens. However with a f/2.8 aperture, you can have more light, which means faster shutter speeds. There are shots that you can take with a f/2.8 lens that you cannot with a "normal" aperture lens. Also, if the subject you are taking is constantly moving, a VR lens does not help much because it only compensates for your handshake and not the subject's movement. To freeze the subject's movement, you need a faster shutter speed... which means either a bigger aperture or higher ISO.
For me, if given a choice of VR or 2.8 (and not both), I'll take 2.8 anyday. I can always carry a monopod with me
Anyway... I was shooting for the Planetshakers + Tim Hughes night of worship... and shooting on stage is fine with a f/2.8, but when going down to the crowd, I had to use my 50mm f/1.4. The other 3 photographers were using the Canon system, and they had the 85mm f/1.2. I tried my 85mm f/1.8 and found that it was still not fast enough. So... to your question... how dark is dark? You can check out some of the shots on my flickr page.
