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They don't even sound the same, except in some (not all) American accents. They don't sound even remotely similar to me.

I thought they did not, but after actually vocalizing them I would say they are basically indistinguishable in real (fast) speech.

Once more I find myself wondering whether English really needs different vowels, if they can all be substituted for each other.

English is easy to speak but hard to write.

Interesting, they sound exactly the same to me, and I cannot even think of how they might sound different to someone else. Care to explain?

Exaggerated, Aaron sounds like "air-ron" and Erin like "ear-ron". It's similar to how some in the US pronounce "pin" and "pen" the same.

Both vowels are quite different. Aaron is /ˈærən/ or perhaps /ˈærɒn/ and Erin is /ˈɛrɪn/.

I suppose that might be true for your accent, but I was referring to an American one since the post was about US boy names.