Encounter suits are supposed to be for practical reasons - to allow a being to live in an environment in which it could not ordinarily survive. However, the Vorlons wore encounter suits for a different reason - to conceal their true appearance. Of course, they had conditioned all races to see them differently, so arguably they were concealing that deception rather than their "true" appearance; still, Kosh was able to leave his encounter suit and rescue Sheridan proving that he didn't need it to survive. He was weakened after this, not because of the exposure to the environment, but supposedly because so many people had seen him.
The purpose of the Vorlon's suit to deceive would suggest then that it was nothing at all like their "true" appearance. But are we talking their true appearance now, or before they "ascended"? The screenshot in another answer possibly shows their current form without any manipulation, but that might not be what they originally looked like. The human encounter suit that you have drawn a comparison to looks a little like human features now, but what does an ascended human look like? Perhaps the idea of an encounter suit for an ascended human is to make their appearance more like their ancestors.