Are there vowels considered as approximants, since some linguists consider /iː/ as high glide? I'm not sure of it but I remember I read an article about it.
You may have seen something like that somewhere, but I would say that is a serious terminological error. "Approximant" is a category of consonant (just as "plosive" and "fricative" are), and vowels are fundamentally different (the primary division, in terms of phonetic classes, is between vowel and consonant). Vowels are not glides. Some glides might be very similar to vowels, for example [j,w] and very similar to [i,u], though the laryngeal glides [h,ʔ] are not non-syllabic analogs of a particular vowel. It has been often claimed that i,j and u,w differ in only one thing, whatever it is that characterizes "syllabic" versus "non-syllabic". It is also true that tense vowels in English are pronounced differently from how they are pronounced in e.g. Finnish which has long and short vowels. This is clearest with "gate, goat" where the vowels sometimes written [e:,o:] are actually more like [ɛi] or, in some systems of writing, [ɛj]. (This diphthongal pronunciation is not universal in all dialects of English). The high vowel analogs would then be [ɪj, ʊw]. So perhaps "the tense high vowels" of English contain a glide.