Because of the shortage of vowels in the Roman alphabet applied to English, the use of "w" as a vowel is crucial.

I am not quite sure what this means. In English, {y} and {w} sometimes do double duty as vowels (/j/, /w/) and semi-vowels (/j/, /u/). Welsh orthography uses both {u} and {w}. In Welsh orthography, {w} is /ʊ/, /uː/, or /w/ and {y} is /ɨ/, /ɨː/, /ə/, /əː/, /ɪ/, or /iː/ (depending on the dialect). Depending on the dictionary, another word with a {w} as a vowel is crwth 'crowd' (a stringed instrument).

Ceci n'est pas un seing.