The Rakghoul plague is extremely virulent and people who are infected can turn within hours. Even if we assume 100% success rate in the vaccine (which is impossible) there are thousands of people on Taris (including pirates and scavengers who don't exactly have Republic support) and what's worse, because of how virulent the Rakghoul plague is, two rakghouls from the same social group can have sufficiently different strains that the vaccine will only work against one of them - if that!
It's certainly not a third-world AIDS epidemic, it's a third-world bubonic plague epidemic. AIDS has the decency of being easy to contain in theory and only spreading through ignorance and carelessness, whereas the Rakghoul plague spreads aggressively through its hosts actively attacking anything in sight.
The Rakghouls can even breed amongst themselves in the modern era (which is how they've thrived despite having no victims to infect for centuries) so exterminating them conventionally is as feeble a gesture as exterminating rat or rabbit populations... except if you screw up here, you'll become part of the problem.
The rat analogy doesn't realy fit.
Most atempts to erradicate rats or rabits (or other varmin) fail, bacause we do not dare to use the most effective weapons at our diposal for fear of unwanted effect on the rest of the ecosystem.
On Taris those constraints do not necessarily exist.
We cannot use acetylcholinestrase inhibitors on rats, beacuse it would allso kill other fauna (and any human who would stray into the aera).
On Taris, the native critters may use a different neurotransmiter than the human descendent Rakghouls, so use of acetylcholinesterase inhibition based neurotoxins may be faisible.