Yes if mRNA works well, we will probably get 95% effective vaccines within the next decade or two targeting diseases such as HIV, influenza, almost any virus basically.
That's not the issue with HIV. The issue with HIV is that it's a retrovirus. It uses both reverse transcription and inscription enzymes to change your DNA.
A vaccine works to stimulate your immune system's "memory" that's evolved for treating infections. (That can be anti-bodies, but it's more complex .. also involves memory T-cells, the complement system, etc.)
When the same virus comes it, it will infect cells, but your body is much more prepared to handle it. The trouble with HIV is that it's the initial infection that can slowly inactivate an immune system over 2~5 years (not everyone though. Some people have HIV and never develop AIDS; known as Long Term Non-Progressives).
A vaccine wouldn't help at all with HIV. Keep in mind, the HIV rapid test checks for the presence of antibodies.
HIV vaccines face tremendous challenges - almost 100% mortality rate if untreated, the fact that immune system activation helps the virus, very frequent mutations, lack of good targets for antibodies, etc. So don't hold your breath, mRNA is unlikely to help much in that case.
Are you speaking from advanced knowledge? My understanding is that you still have to find a relatively stable binding site that doesn't have a high degree of similarity to human cell markers. So the mRNA technique is a great path from the identification of a binding site to a vaccine, with the difficulty of that identification not being particularly predictable.