I encountered the word "Typ" (en:type) which has a 'y' serving as a vowel; in this case a 'ü', I believe.
Is this a singular case or are there more words in which 'y' is used as a vowel?
Essentially, the letter "y" itself is no vowel in the German Language. The letter that are vowels in German are:
(On Wikipedia there is also mentioned É == ee, as in "Varieté" .)
Moreover, they WIKI article says that the letter "y" can be accounted as a vowel, since its pronunciation is often like the German "ü". In some circumstances it is pronounced as "j" (*), but in some cases that's true for "i" and "u", too (e.g. Qualle [ˈkvalə])
Strictly speaking, we have to consider the way how we pronounce the letter. That is, in "Typ" the vowel is a long ü.
(The bold part is the vowel which covers "Typ".)
For that reason, we can count "y" as vowel. Though, note please that the y just occurs in adopted words from different languages. Words that are borrowed from Greek are pronounced as "ü". In all other cases the "y" is pronounced as in the "original language", therefore it also can be pronounced as "i" (Party, Hobby). WIKIPEDIA
In a nutshell: Besides of proper names there are no German words that have the letter "y". Thus, for the German language itself it is hard to say if it is a vowel or not. For the words from foreign languages we take the pronunciation of the other language. That don't makes the "y" a vowel in the German languages automatically, not least because there are a lot of "exceptions" where it isn't pronounced as vowel. I don't see "y" as a vowel of the German language and that's what you learn in school, too.
(*) I just want to mention a few words in which the y is spoken as consonant j:
For the sake of completeness, I shouldn't forget to mention diphthongs:
A vowel is really primarily a type of sound rather than a letter. But the letters used to represent vowels can also be referred to as vowels (Vokalbuchstaben in German). In both English and German, Y sometimes represents a vowel and sometimes a consonant.
It looks like Y was originally introduced into the Roman alphabet solely for the purpose of transcribing the letter Y (ypsilon) - which is a vowel - in Greek words. See the following quote from Wikipedia (from the German page on the Roman alphabet):
Since Y is called Ypsilon in German, and based on the fact that the only reasonably old German words I know are actually Greek, I'm guessing that for a while after the Roman alphabet began to be used for German, Y was only used for transcribing Greek words (and therefore only as a vowel), as had been the case in Latin. But this is only a conjecture.