If you're paying the full cost of the vision insurance yourself, it's almost definitely cheaper to go without, unless you absolutely MUST buy your glasses from a traditional high-cost optician. And even then, it might be cheaper to go without.
Call around and find out how much an eye exam costs without insurance. I think for me it's something like $100-150 (including the exam for contacts). I buy my glasses for $100-150 from Warby Parker and they last me two years or more (my prescription doesn't change much). I have, in the past, bought glasses online for as little as $30, and they were fine (and they had anti-glare coating). I buy my contacts online as well, in bulk for a small discount (I wear dailies because I only use them occasionally).
So for me, every two years I would pay maybe $150 (exam) + 100 (glasses), not including contacts (which cost pretty close to the same everywhere, with or without insurance). Even if I spent $200 on glasses, I would still be below what you would be paying for the exam copay + premiums with your insurance.
The reason vision insurance is such a blah deal is because the costs are very predictable and only people who need it buy it. Full health insurance costs are highly variable and potentially catastrophic, so lots of people buy health insurance and mostly don't use it. Vision insurance ends up being more like a payment plan than true insurance. Also, a lot of the vision insurance plans are owned by the same companies that sell the glasses/contacts!
Oh and finally: I would say get new glasses (even if they're nearly the same!) rather than getting new lenses for existing frames. It's only a little more expensive and then you have spare glasses and you don't have to give up your glasses and worry that they're going to break them (they broke my favorite frames).