In short, no. Your company is not required to offer dental coverage for adults under the ACA.
While you're not required to offer vision coverage for your adult employees, you may be required to provide it for any children under 19, including family members of your employees.
Your company is allowed to have a "grandfathered" plan that wasn't purchased through a government marketplace if your business has more than 50 employees, but "small group" plans for companies with fewer than 50 employees are subject to the standard of "essential health benefits" set out by the ACA, and that includes pediatric dental.
Small group health plans that are ACA compliant now include pediatric dental. This meets the requirement for offering dental coverage to children. However, many plans include the option of adding dental coverage for adult employees, and your company is free to purchase those plans if it wants to do so. In addition, offering dental coverage can boost employee well-being by enhancing employees' ability to take care of their teeth and make your company a more desirable place to job applicants who assess a company based on the variety of benefits that the company offers.
ACA Employee Answers - Get answers to common employee questions about the Affordable Care Act.
ACA Business Answers - Get answers to common small business owner questions about the Affordable Care Act.
Small Business Insurance Marketplace - Find out what options small businesses have for purchasing insurance through a government exchange.