"To date, no Human has ever seen a Vorlon."
The Vorlons were one of the oldest races in the known universe. Vorlons are known to be immortal; they can not die of natural causes but they can apparently be killed. Vorlons have a energetic/squid like appearance outside their encounter suits, looking as if they evolved into an energy/plasma based life form. However, due to genetic manipulation by the Vorlons they can appear to have an angelic appearance to other races, with a variation on each race. Vorlons also seem to be telepathically linked and have the ability to enter the minds of other people under the right conditions. All Vorlon technology is also organic based.
Now as for the effectiveness of the Vorlon "Encounter suit", it protects the occupant within a virtually impenetrable organic shell. When forced to kill Ulkesh, it took direct energy from all of Babylon 5's power resources, combined with the PPG fire of an entire attack squad just to drive the Vorlon Ambassador from this shell. Still, despite it's resilient nature, the encounter suit is susceptible to other forms of attack.
When the original Ambassador Kosh first arrived on Babylon 5 in 2257, a Minbari impostor succeeded in actually poisoning Kosh directly through his Encounter suit when they shook hands. It may be assumed that this poison was not stopped by the Encounter suit, as the poison patch was connected directly with the Vorlon's hand. At the same time it is generally assumed that the Vorlon's hand was in fact covered in a thin enviro-bandage to prevent the loss of atmosphere and contact with bacteria/viral agents through the pores. It is more probable that the hands were in fact covered by a thin layer of material which was not gas permeable. As for the poison tab, we must either assume that the suit allows liquids to penetrate, or that the poison was specifically designed to penetrate the suit. This is acceptable, as the Minbari did know quite a bit about the Vorlons.
As Vorlon technology is organic in nature, we are leads us to assume that the Encounter suit is actually a living organism. Thus the Encounter suit probably absorbed the poison, and either by system failure or by the virtue that some/most/all of the Encounter suit is really just to hide the identity of the occupant, that the poison was transferred directly into the blood/plasma stream of Ambassador Naranek, causing his illness.
Now that the Vorlons, as well as all the other first ones, have passed beyond the Rim, it is unlikely we will ever see them again until we ourselves are ready to pass.