Prefixes become part of the word and figure into the stress patterns of the words they are attached to. This sets them apart from prepositions and other words that cause mutation. Because they become part of the word they are attached to, we're adding syllables to the words. With added syllables comes vowel reduction.
As with the simple plural, we will break these changes down to Final and Non-Final vowels/diphthongs.
If a word has more than one syllable, the last syllable will not have a long vowel in it. There are a few exceptions in some compound words, which are presumed to have been made after this sound-shift swept through Sindarin.
This means if you have a single-syllable word with a long vowel, that vowel is shortened all the way down, no matter what vowel it was.
The only diphthong that changes is AW/AU. It becomes an O.
If a word has more than one syllable, then the vowels in it have already been reduced. All there would be left is a long I, O, U, or Y. Therefore, there is nothing left for you to reduce!
Ran "to wander" + (n)dîr "man" → Randir "wanderer"
Al- "not" + taw "woolen" → Al-do "not woolen"
Pen- "-less" + taur "king" → Pen-dor "kingless"
Al- "not" + írui "desirable" → Al-írui "not desirable"