That's a shame. It's why I try not to use brand new stuff on an important job without some testing first.
Cheap, uncoated filters can cause focus errors... Circular Polarizers in particular have two layers of glass, separated slightly by a little space. So there are four air to glass boundaries for light to cross, or twice what most other filters have. So it's particularly risky to use low priced C-Pols.
I don't know about that particular filter... I'd be suspicious, though, of a $40 C-Pol filter, just based upon the price. In 67mm size, the B+W Kaesemann C-Pol that I use (in other sizes) sells for $128.
Vivitar doesn't actually make anything themselves. They just buy things from other manufacturers and have their name imprinted on them. Who knows who made that filter.
It can help to use a lens hood. Did you?
That shot is so OOF, I gotta ask, are you certain it's a circular polarizer, not a linear?
It is possible that the filter in combo with your lens was the problem. An f4 lens effectively becomes an f5.6 or slower, once a C-Pol is added. The filter costs at least one stop of light. Most Canon other than the 1D series you need to use only the center AF point, if the lens is slower than f5.6. And even that will be slowed. Still, I would think the 7D should be able to handle it. I've used manual aperture lenses with mine and noticed that they really slow down a lot at f8 and can't give focus confirmation at all at f11.
