Tamron PZD 18-270 issue using Vibration Compensation

This is normal and will occur for all stabilization systems.

Stabilization corrects for aiming errors due to shake. However, stabilization can only correct for a certain distance of movement. If you have a shake that exceeds this limit, what should stabilization do? One option is to recenter the image (this results in a ghostly double image). Another is to keep the correction pegged as far as it can go (leading to a sharp image with a smear on top of it).

This is what you saw. A camera shake that exceeded the stabilization's ability to correct will blur the image worse than had you not used stabilization at all. Tamron's VC locks the viewfinder so well that you can't gauge how bad your shaking is, so you could be shaking too much and not know.

Another possibility is that you fired the shutter before stabilization had time to settle. Earlier Tamron VC units tended to cause the image to "jump" when engaged. If you mash the shutter button from not-pressed to fully-pressed, you could actually catch the lens mid-jump and get a very blurry image that way. The solution to this problem is to half-press the shutter for a half second or so before taking the picture. For some, this is unacceptable, for others, they anticipate the shot (e.g. half press while raising the camera up to the eye). Ken Rockwell refuses to do this and thus dings VC lenses pretty badly in his reviews. Nikon VR appears to not have this issue (although it's debatable as to whether VR is fully effective if you mash the shutter button).