A few thoughts:
I don't know if ENT weighed in on this, but I seem to recall when Data was doing one of his verbal data-dumps about comparative views on mating that he said Andorians married in groups of 4. Trek Lit seems to have taken that ball and run with it to the idea that Andorians have 4 genders. If that is the case, I doubt they could interbreed with any of the 2-gendered races.
If, however, you decide that Andorians are 2 gendered but have a social convention of polygamy (perhaps requiring 2 males and 2 females to make a "marriage" in their culture), then cross-breeding is more likely.
Trek has cross-breeding a lot more likely than reality, probably because it's cool.
So let's roll with that.
I believe the only time we have had an on-screen mention of a crossbreed that wasn't "Human-something" it was Klingon-Trill. Trek seems to follow the convention of many Fantasy settings that humans can breed with almost anything. So, again, there is reason to doubt an Andorian-Vulcan is possible.
But you didn't ask if it was possible, you asked what it would look like.
From what we've seen (with the exception of Spock), half-breeds tend to be a blend of their parents' features. So:
Pointy ears, though not quite as pointy as a Vulcan's (see: Simon Tarses)
Antennae, though smaller than most.
Slightly paler blue, perhaps a bit aqua.
Hair white or black, or a mix (like salt-and-pepper: some strands white, others black)