What is the literary term used to describe long vowel sounds?
For example in Ted Hughes's "Your Paris" in his Birthday Letters anthology "Eerie Familiar Feeling", what term would be used to describe this literary long vowel sound technique?
The only literary technique exhibited there is alliteration, but that is not limited to long-vowel sounds. It refers to similar sounds placed close together. In this case eerie and feeling both have the ee combination, and familiar and feeling both begin with the f sound.
Now, if you are really asking about a term to describe long vowel sounds apart from alliteration, you are asking a question about linguistics, and that is not really the topic of this site.
Some have questioned my usage of the term alliteration here. I maintain that it may be used in the broader sense described above. The use of consonance and assonance to refer narrowly to consonant and vowel repetition is a distinction, not a generally restrictive provision. Alliteration can also refer to phonemes with similar properties, anywhere in a word.
When I studied Old English, in which alliteration reigned absolutely, we were taught to consider a line like this one from "Cædmon's Hymn",
as containing alliterative matches in "ærest", "eorþan" and "bearnum": ær, eor, and ear. That means the definition included vowel sounds that were near matches, and also occurred elsewhere than at the beginning of the word.