1 Lambert: It’s Juiblex!! I told you not to invoke the name of any demon lords!
2 Mordekai: I wasn’t invoking! I was just saying the name as part of your silly game!
2 Lambert: Same thing!
3 Mordekai: It isn’t identical! Invocation implies intent!
3 Lambert: Idiot! Invocation is implicit!
4 Juiblex: {thinking} Normally I absorb mortals for their intelligence, but I’m thinking twice.

The word "invoke" has several related but slightly different meanings. According to dictionary.com:

• to call for with earnest desire; make supplication or pray for: to invoke God's mercy.
• to call on (a deity, Muse, etc.), as in prayer or supplication.
• to declare to be binding or in effect: to invoke the law; to invoke a veto.
• to appeal to, as for confirmation.
• to petition or call on for help or aid.
• to call forth or upon (a spirit) by incantation.

The one relevant to this comic is the last one. There's another thing about this comic too:

Interestingly, I intended illuminating its idiosyncratic initial "i" insistence, including impeccable intellectual information.

Now, I know enough to know that alliteration is supposedly, according to most scholars of literary devices, the repetition of consonant sounds at the beginnings of words, and that assonance is the repetition of vowel sounds. However, assonance is not restricted to vowel sounds at the beginnings of words, and in fact more often refers to the repetition of vowel sounds within words. So if you listen to the literary terminology pundits, what we have here is not alliteration, and only an outlying subset of assonance. Repetition of vowel sounds at the beginning of words doesn't seem to have any specific word to describe it.

So I hereby propose that we call it mordekation.

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Richard B. writes:

You're off the hook as far as alliteration goes! I read a bit of Middle English (found a pocket edition of Chaucer in my student days, and took it from there) as well as Middle Dutch, and I do a bit of fixed form poetry, and from both sides, I can tell you that yes, alliteration is when words start with the same consonant. However, no consonant also counts as "a consonant"! That is, all words starting with no consonant, i.e. with a vowel - any vowel - were often considered to alliterate! Not always, true, but quite often.

Thus, if all your stressed words started with a vowel, that was acceptable. Of course, in those days, alliteration only mattered in poetry, and it only mattered if the stressed syllables alliterated. Alliterating a random set of words from a prose fragment wouldn't have occurred to them. Still, if you want to alliterate on vowels, you can - and you don't even have to stick to the same vowel. Any null-consonant will do.