A couple things:

1) We generally know the strength of the placebo effect and the rate of HIV transmission among at-risk groups, so we can control for it, and

2) Not sure the placebo effect is particularly relevant. First, a vaccine that shows little to no benefit over the control (placebo or not) is not an effective vaccine. Second, placebos aren't effective for everything. I highly doubt the effectiveness of a placebo at preventing HIV transmission (although it would be difficult to test ethically).