Making a vaccine is for achievement of introduction of the agent to the body. Body reacts to new, vaccine, agent primarily... which is mild. The subsequent attacks are handled vigorously. As body acts specifically for specific agents, you need to keep in mind how you making the vaccine will affect the receivers. Basically, briefly speaking... vaccine is nothing but part/whole of the disease agent, modified to not make receiver ill but good enough to introduce nd initiate a host defence mechanism. HIV directly targets the immune cells, and turns them into a factory of new infected cells, so to find a safe way to introduce it into a human with controllable outcomes, it'll be a complex process... moreover, HIV is an RNA virus, which means the mutation rate among HIV offsprings is frequent, that mean one vaccine made for a type of virus may/may not be effective against the mutated virus now... Okay this is the simplest i could make it😮‍💨