I know it's not much, but 26 vials seems like a lot. It is, unfortunately, taken at one visit.
The goal of the vaccine study is to determine if the body can create antibodies to the HIV virus. So throughout the study there will be tests to determine the levels of antibodies. This is simply a research vaccine and will never be used amongst the general public. The hypothesis the research team is working with is that a body with HIV antibodies will be better equipped to fight off the infection if one becomes infected.
There are two types of vaccines. One type, like smallpox, will keep you from (hopefully) from ever contracting the virus. On the other hand, there are vaccines, like the flu vaccine, that may protect you from some strains and, if you are exposed, the symptoms may be less pronounced. The HVTN 505 vaccine falls into that category.
Like many clinical studies, it is a controlled study. There is an experimental group and a control group. The study is double-blinded until the research is completed, meaning that neither I, nor the researcher knows if I'm receiving a placebo.
I hope this helps to clarify things a little. If you are interested, you can find the study information on the HVTN website or through the NIH.