Yiddish has many words from Hebrew, which are spelled to Ashkenasian custom, without vowels. For all other words, whether from German or other languages, some Hebrew characters are "abused" to represent vowels, quite phonetically, so that different dialects will result in different spellings.

Note that nowadays Yiddish language is used by non-native speakers and writers to transport political and ideological content, similar to, say, the Goidelic languages. We cannot criticize this altogether, but must be aware that it does not come from an uninterrupted tradition.