There is already prep, a once a day pill that if taken consistently, is over 95% effective at stopping HIV infection, I've heard some sources say it's 100% effective. While it's not medically considered to be a vaccine, most vaccines for other conditions have a lower effectiveness rate. There is work being done to turn prep into an injectable chip in the arm, like long term contraceptives, but for HIV prevention.
The thing that worries me about antiretroviral drugs is that they mess with mechanisms that are innate to the reproduction of cells. Their effects can, e.g., be clearly seen in changes to the bone marrow, which IIRC has some of the fastest-reproducing cells in the human body. We don't have long-term data yet on what this does to users of this drug. My wife has to take such drugs as she has HIV, and I wouldn't be surp…
But it's quite a tricky decision to make. Vaccines aren't without risk either, though adverse reactions are also rare. Certainly from a cost perspective, PReP with high efficiency makes a lot of sense – lifetime treatment and monitoring of an HIV+ patient can be markedly more expensive than a relatively cheap preventative medication, especially in high-risk groups.
I have been beating around the idea of taking PReP for a while, what with being part of a higher-risk group in an area with relatively high HIV prevalence. It's definitely a harder commitment to make than vaccination is, but I hope that we can improve our knowledge and understanding of the risks as the trials currently underway progress.