Medicine is not based on what you think or assume, it is based on evidence. There are plenty of treatments out there that we do not really know the mechanism behind their action, and yet we use in medicine due to the evidence that supports the fact that they are capable of improving a certain condition. As to your logic, our immune system mounts a very strong reaction against HIV, and is capable of clearing our plasma almost entirely. The problem lies with the fact that HIV manages to camouflage itself inside immunological cells such as macrophages. The cells responsible for mounting cellular killing are T-Cells, the very same cells HIV eradicates slowly over years, and that is the main reason why you see the viral load jumping back up after you halt protease inhibitors. If a vaccine manages to stimulate both the T-Cells and the B-cells strong enough to have an effective response that could prevent infection, a functional vaccine is very possible.