Vaccine basically means any person who takes it of any age, gender, any climatic conditions etc should be cure the disease and should not have any side effect. So, this is what is taking time and as you mentioned that till all this condition are to fulfil WHO is not going to approve it. Hoping soon things turn out to be better for the world.
I don't agree with the first part. Vaccine does not cure any medical condition. It is a preventive medicine. There is no point in taking the vaccine, in case you have already become infected. In that case, you need to take the medicines. The normal vaccine approval process takes anywhere from 10 to 20 years, as there needs to be studies for long-term side effects. In the case of COVID 19, they are going through a hastened approval process. Even after all that, it can take up to 2 years for the process to be complete.