Are there any proven methods to strengthen vision?

Unfortunately, no. People with nearsightedness, farsightedness, astigmatism, or presbyopia (the need fro reading glasses) are not experiencing a disease. These are related to the shape of the eye or the natural changes occuring in the lens, and can only be treated by correcting the prescription (glasses, contacts, lasik, catract surgery). Don't lose money on products that claim to work - they don't.
No. An eye is not "weak" if it can be made to see 20/20 or better with glasses or contacts. If an eye cannot see 20/20 with glasses or contacts (and there is no other eye disease to explain it) there are no proven methods to "strengthen" vision.