True Veneers which are .5 to .7 millimeter can improve and whiten the original underlying color 25 to 30%. Basically a true porcelain veneer can improve the underlying dentin, just as the enamel blocks the yellow color of the underlying dentin. If you had thin veneers and not regular porcelain ...
In order to fix a deep overbite, you need to see an expert dentist near you for a complete evaluation with xrays,models, and comprehensive exam.
You usually can have deep overbite correction with orthodontics, the other way is through building up the back teeth, dental reconstruction, in which...
I would pay a visit to your dentist to have the veneer checked out and replaced if necessary. A small crack may become worse over time, so it's better to have it looked at sooner rather than later.