w as a vowel

It's a good idea to separate the orthography (spelling) of a language from its phonology (the actual acoustic and articulated sounds of a language. Welsh orthography uses w to represent /ʊ/ and /uː/ (which sounds exist in English put and through). Welsh w can also sometimes stand in for a semivowel, as in English wet). Some Northern dialects of Welsh do have some vocalic sounds that do not exist in English. The are central high vowels and are represented in the orthography by u.

People will say things like English has 5 (or 6 or 7) vowels, but they are talking about spelling there. Most varieties of English have over 10 distinct, monophthongal vowel sounds. Unfortunately our "system" of spelling has the 5 cardinal vowel symbols to work with.

Ceci n'est pas un seing.