Quote: If I can chime in with some thoughts:
Firstly referring to Nikon's VRII system, users of quite a few VR II lenses have been complaining of softness in images around the 1/100-1/200 sec exposure time, I have also noticed it with my AF-S 70-300mm VRII lens with these speeds and especially at 1/160 sec. the image is blurred slightly, longer shutter speeds are better down to 1/60 and above 1/200 they are also sharp.
The (half) technical explanation is that the VRII system runs at (I think it's) 300HZ and certain shutter speeds have some sort of conflict with the frequency of the VR, Nikon received many returned lenses especially the AF-S 300mm F4 fresnel lens and were unable to improve the situation by firmware updates.
Eventually they issued a statement to the effect that, "due to the way the system functions some users may experience blurring at certain shutter speeds, these speeds should be avoided"!
I have and use regularly the Tamron 150-600mm VC for the most part it's system functions fine at shorter to medium focal lengths, however at the long end the systems seems to be over zealous in it action of trying to hold steady the image, till finally it let's go and restarts again. This means when panning at the long end most of the shots are blurred and the only remedy is turn the VC off!
This effect of over zealous VC is also noticeable as a certain jerkyness in video when panning with many Tamron lenses. (see Christopher Frost's utube videos)
I would suggest trying a series of test shots with a range of shutter speeds to try and find the speed where the problem is the most troublesome and avoid them, à là Nikon! If that doesn't resolve the situation you could either leave the VC off or return the lens to Tamron,
Your version was one their first ventures into stabilized lenses.
I can confirm that the Tamron 150-600mm's VC struggles beyond 500mm. First I thought it was a defect but then others reported this as well. The Sigma 150-600mm S has no issues here.