At http://vspdirect.com/ Davis Vision, the other large vision insurance carrier also has a direct purchase plan.
I bought the dental plan for my husband and me through eHealth (http://www.ehealthinsurance.com/?allid=eHe29411) since it was the easiest way to locate a plan that fit what my dentist recommended. eHealth also offers vision plans but I had already signed up for VSP when I found their site.
I was surprised that our vision and dental coverage cost about the same as we had been paying through my husband's employer. I'd always assumed we were getting a discount through their group policies. If I'd known buying direct cost about the same when they switched to Davis Vision, I would have opted out of their vision policy and bought VSP directly since Davis didn't cover the eye doctors we had been using for years.
Both VSP Direct and eHealth let you check providers and coverage on their sites, so that is a major improvement over the coverage through the employer which doesn't let you know until you are already signed up for the next year.