Both vaccine and non-vaccine groups receive appropriate counselling (this part can be double blind even) regarding HIV, risk behaviors, and as a trial vaccine it might no be 100% effective etc etc... You assess the impact of behavior changes via surveys and interviews to see if there has been a significant increase in risk behavior and the control for it.
It would be unethical to give a placebo to someone for this kind of disease.
Until the treatment is proven - it may turn out to be the case that the placebo is more beneficial than the treatment.
If you knew that it would be "unethical to give a placebo to someone" then you wouldn't have to do the trial!
Indeed - there have been situations in which the Trial turns out to be so positive - that the study is canceled, and all participants are given the treatment. I think that was actually the situation with the circumcision trials - circumcision turned out to be so ludicrously low risk, and so effective at reducing HIV infections in heterosexual male populations in Africa, that they, ethically, had to prematurely end the study and circumcise all the participants.
It would be unethical to give a placebo to someone for this kind of disease.