It is entirely right, always 'an' before a vowel, not 'a'. There are other rules for when 'an' is correctly used before a consonant, I just didn't expand on them as you did.
By "not entirely right" I didn't mean you were wrong, but that you had left out the important part in that its based on vowel sounds, not on vowel letters. There are some rare cases where a vowel letter is silent or such where a vowel letter doesn't designate that there is a vowel sound there, thus meaning "an" would not be used in this case. I can't think of an example since it is such a rare case.
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