Given the highly persistent nature of rabies after entering the brain, I was wondering whether certain ways of exposure to this virus could be risky even for a previously vaccinated individual. I understand that no vaccine is 100% effective, but would the risk of infection be particularly increased in case of exposure at some particular site on the body? Which mechanisms of the rabies infection would play role in this situation?
Reframing your question as:
Are rabies exposures at specific sites on the body riskier than other sites, even for a vaccinated individual?
Possibly for post exposure prophylaxis, but probably not for pre exposure vaccination.
Rabies transmitted by a bite or scratch (almost all cases) involves a period of slow replication in muscle tissue at the site of the bite or scratch (see Murray Medical Microbiology Ch. 60 for a general discussion). This is called the incubation phase, and it is highly variable. The incubation phase is followed by a prodrome phase (virus binds receptors of sensory neurons at the site of exposure, and travels to the CNS) and neurological phase (infection of the brain).
An effective immune response is believed to be the result of both an antibody response and cell mediated immunity during the prodrome phase. Post exposure vaccination is effective because the incubation period provides enough time for a naive immune system to mount a protective response (same reference). Regardless, rabies post exposure prophylaxis (PEP) includes both active and passive immunization, i.e., vaccine and human rabies immune globulin.
Both post and pre-exposure prophylaxis failures have been reported. As discussed here cases with pre-exposure vaccination failure involve immunocompromised individuals. Cases with post-exposure prophylaxis tend to involve failure to fully follow guidelines (some are summarized here), but there is a recent case where guidelines were followed. These cases tend to involve bites or scratches to the head and face, which tend to involve a more rapid progression from incubation through prodrome to the neurological phase.
So, since reports of post exposure prophylaxis failure tend to involve specific locations of exposure (head and face), it's reasonable to assume that an exposure there has a higher risk. There is no data supporting this risk for cases with pre-exposure vaccination.
What a vaccine does is simply providing the body an attenuated (avirulent) form of the pathogen, in this case Rabies. What our immune system does is recognise the particular foreign substance and produce immune cells against it, of which some cells remain as memory cells which would trigger an immune response much faster if the same pathogen enters the host again (assuming that the vaccination was successfully performed).
However, as you know that many virus mutate very quickly and if the extent of mutation leads to production of new rabies virus which is highly distinct from the form that was used as vaccine then the host will not be able to identify the new virus as rabies.
In that way the infection may take place