Diabetes can affect your vision in more than one way. The vision is dependent upon sugar levels in the blood. If the sugar level is too high your vision may be corrected with a certain prescription but that may change coinciding with the change in blood sugar levels.
The other way that diabetes affects the vision is by affecting the health of the retina. High blood sugar levels affect the wall of the vessels throughout the body. In your eye this means leaking of the vessels may cause hemorrhages which may cause damage to the retina and permanent vision loss. Control of sugar levels is key to long term stable vision and health of your retina.