Short answer: Look at the exam copay, the frame and contact-lens allowances, lens and lens-upgrade coverage, how often you can get new lenses or frames, and the provider network.
A vision plan’s value comes down to a few features: the eye-exam copay, the allowance toward frames and the separate allowance for contacts, coverage for lenses and add-ons (like progressive or anti-reflective lenses), the frequency limits (how often you can get new materials), and which retailers and doctors are in network. Allowances and copays are richest in network, so check that your preferred provider participates.