I don't think need to replace the belt, unless it is faulty or keras or even longer already
Thus, i believe you may tighten the belt, maybe it will solve your problem as well.
The problem is due to the material of the original belt. Tightening the belt may not solve it, and over tightening will cause undue stress on the belt (can snap). There is a recommended torque on it (you can refer to the service manual).
Seems that the newer Vios does not have this problem anymore.