The aim of this page is to explain the norms used in Volapük entries. It is intended to complement, not supersede, WT:CFI and WT:ELE. Volapük is considered a well documented language, which means that all senses must be attestable, so that it is possible to find at least three independent attestations spanning at least a year.
The letter combinations 'ay', 'oy', 'uy' should not be considered "alternative spellings" of the characters 'ä', 'ö', 'ü'. Quote from the Wikipedia article:
Note: ä, ö and ü do not have alternative forms such as the ae, oe and ue of German.
Also, 'y' is used legitimately in Volapük as a glide consonant, so using 'y' also as a crutch substitute for umlauts would disrupt Volapük's phonetic spelling.