1/30 at 280mm is a recipe for blurry for anything moving.
1/30 is slow enough that normal "people movement" is often perceptible even at normal focal lengths, even when sitting still. Long focal lengths like 280mm multiply the effect of subject movement due to the magnification inherent to the focal length. At a wide angle, a person might fit entirely into the image (i.e., a full body shot)... take your image size to be an 8"x10" print. Look at what one inch on the print represents, at the plane of focus. It could be 6" or greater. Maybe that 1" matches the width of their head, or the length from their fingertip to their wrist, or the distance between buttons on a button-down shirt. Now look at an image taken at a long focal length, same subject distance, same print size. It is now possible that their head is taking up most or all of the image. That same 1" on the print might be the length of their nose, or the width of one of their eyes.
Now back that out to consider what "blurry" is. Let's simplify and say that it's a pixel of difference. (Look up "circle of confusion" if you want a more detailed discussion of how blurriness is defined in photography.) If a person sways 1/4" during the time of your exposure then it might not be perceptible at a wide angle, if that 1/4" takes up less than a pixel. But a long zoom will make that 1/4" take up multiple pixels and result in a blur. So it is all relative to a lot of things, including the pixel density of your sensor.
VR can correct your camera shake pretty well, but again this camera shake is magnified by long focal lengths. If your camera shakes 1mm during your exposure, this subtends a small arc (relative to the size of the image) at wide focal lengths, but it might be the equivalent of a sizeable fraction of the image at long focal lengths. VR is pretty good and can correct for much of this. I would say that 1/30 at 280mm with a Nikon VR lens should be possible for a completely static subject, if you have good technique and you're not being blown around by the wind or standing on a vibrating vehicle or something.
In the end you only have 3 variables to exposure: ISO, shutter speed, and aperture. Ironically, by being zoomed in so much, you hamstrung yourself with both shutter speed (because the shutter speed required to take a sharp photo of a moving subject decreases with the focal length) and aperture (because the maximum aperture of your f/4-f/5.6 lens gets smaller (numerically larger) as you reach the longer focal lengths). Your only recourse is to high ISO. Indoors at over 500mm equivalent, I would definitely try not to go slower than 1/100 shutter speed, and of course keep your lens at its maximum aperture (which also generally decreases sharpness, BTW -- most lenses are at their sharpest when they're stopped down a stop or two. This probably isn't a major factor in your case, but it contributes.). Bump your ISO as high as it needs to go to get the right exposure, or as high as it can go if it can't go that high.
BTW f/4 lets in twice as much light as f/5.6. f/2.8 lets in 4x as much light as f/5.6. So under the same conditions and camera settings, with an f/4 lens you would have had 1/60 shutter speed, and with an f/2.8 lens you would have had 1/120 shutter speed.