In the last episode of season 4 (The Deconstruction of Falling Stars) the story jumps forward a million years and shows an evolved human in an encounter suit.
The head of the encounter suit is very similar to a human's head. Would it be correct to suppose that the head of the Vorlon's encounter suit is also similar to a Vorlon's real head before they evolved?
No.
There is an episode, "Falling Towards Apotheosis," also in season 4, where
the Vorlon's true appearance is actually depicted, and they are clearly not bipedal at all, with no obviously discernable head.
No, the suit is merely to hide their true form which appears more ethereal. Each race views them differently. This was explained in the episode where Kosh saved Sheriden from falling from the transport car. Each race views a Vorlon as a deity from their planet's culture.
Further infomation can be found here - https://babylon5.fandom.com/wiki/Vorlon
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.
