I think they tested a whole bunch of vaccines and found one that actually worked. They still have no idea how it works yet. Like the article said, the HIV virus is a mutating virus, just like flu. The way our immune system works is that it churns out humongous amount of immune cells (B- and T-cells) that have various antibodies attached. The goal is to "mark" the invading cells (be it virus, bacteria, or pollen if you have allergy). The hope is that one of those antibodies will match SOME unique features of the antigen and put a label on the invading cell. These labeled invading cells then immediately become targets of attack by our own killer cells, like macrophages. These killer cells either eat the invading cells alive, or secrete peptides that drill holes into membranes of the invading cells. pretty gruesome stuff. So the key for all this to work is to find a B- or T-cell that has the matching antibody that can actually label the invading cells. Unfortunately, there is no easy way out of this. Our body simply churns out as many of these cells as possible and hope one will match. The way I think of it is like a locksmith trying to open a lock by making vast amount of keys and hope one will match the lock. Not the most efficient way, but it works.
Many viruses get away by constantly changing itself. If the key of our immune systems is to find a matching key, imagine what would happen if a locksmith is trying to open a lock while the lock itself keeps changing. So our immune system always lags behind a little. That's why we still don't have an effective way of curing something so simple like a common flu and so terrible like HIV. Doesn't matter what drug you come up with, the virus keeps changing! That guarantees some, however small, population of the virus will survive. And give it a couple days/months and millions of generations of viruses, you will have another pandemic in your body.
That's where vaccine comes in. We are introduced with a small population of either dead whole viruses or parts of viruses. This way, we won't get sick while giving our body a chance to manufacture "labeling cells" that match the virus. And our body would then store these matching cells for rainy days. The idea is that we would have these matching cells ready to label viruses as soon as they attack and give them no chance to mutate. The difficult part of this, in the case of flu virus, is that there are hundreds of types of flu virus and we have to have matching cells for each type of them. that's why we have to get flu shots every year. Another problem is that these stored matching cells have shelf life and expire from time to time.
The problem with HIV is that it changes so fast and it's a retrovirus, meaning it injects its own RNA into our own cells. Then our own ells turn these RNA into DNA, which combines with our our own DNA. Then WE ourselves start pumping out viruses. Even if you kill the actual virus, the viral DNA is already in us, much like in the movie "Alien".
One key step in viral infection is the fusion of viral membrane with host cell membrane, which then allows the virus to dump its RNA into host cells. A major trend in the field is to figure out a way to stop this fusion. We actually did some theoretical work on the the process of membrane fusion, which is NOT well known at all.
Sorry about the long post. I get excited whenever people talk about this stuff