“Cwm” (a steep-walled semicircular basin in a mountain, sometimes containing a lake; a cirque) and “crwth” (an ancient Celtic musical instrument), both from the Welsh, use w as a vowel – standing for the same sound that oo stands for in boom and booth. “Crwth” is also spelled “crowd.”
However, in words like “low” and “bow,” one can make a good case that the letter w represents a vowel. Both of these words end with one or another of the diphthongs of modern English. In each case, the second part of the diphthong is represented by w.
By the way, l, m, n, and r may also sometimes represent vowels; that is, in English there are vowels that are routinely represented by these letters. They show up at the ends of the words “bottle,” “bottom,” “button,” and “butter.”