As I understand it, existing HIV medications need to be taken consistently, forever, in order to be effective, and there's still a (really tiny but extant) chance of still being contagious? Uncertain on that last part.
However, you still need the drugs constantly once you've been infected. If a vaccine could be provided that eliminated the need for sustained medication (and thus the risk of loss of access, which can be pretty high in some African countries, I'd imagine), you would have a treatment that objectively is better for someone's quality of life.
So I guess you'd ethically be weighing the risk of misbehavior of the experimental medication causing damage to life versus the long term, sustained risk of loss of access and associated damage caused by just sticking with and improving the existing treatments.
I'd much rather see a population given a one shot treatment than to be permanently beholden to the whims of a pharmaceutical company. Sure, they're BOTH from the same company, but one's scarier than the other in my mind.