Yiddish has an unusually small vowel inventory for a Germanic language, which are generally notorious for their large number of vowel phonemes. Probably under the influence of the surrounding gentile Baltic and Slavic langauges, Yiddish contrasts only five full vowels (/ɪ ʊ ɛ ɔ a/), a marginally phonemic reduced vowel ([ə]), and two or three diphthongs (/aɪ̯ ɔɪ̯/, with /ɛɪ̯/ only contrasted in some Lithuanian dialects, corresponding to /aɪ̯/ or /ɔɪ̯/ in the others, and re-introduced in English loanwords). There is also no phonemic vowel length, except marginally in dialects where /aɪ̯/ can sometimes be realized [aː] or /ɔɪ̯/ as [oː]. These transcriptions are broad--point is, a small quality-based 5-point cardinal vowel system in the middle of a famously vowel-happy language family.
I want to know, are there any other Germanic lects--language, dialects, etc.--which contrasts as few vowels as Yiddish does, or even fewer? I don't want to outright make the claim that "Yiddish has the fewest vowel phonemes of any Germanic langauge", because I'm no expert on Germanic dialectology. Does anyone know of any Germanic languages with vowel systems as simple as Yiddish or simpler?
EDIT: Qualified the 'no phonemic length' claim to note that some dialects do have [aː] and [oː] as realizations of /aɪ̯/ and /ɔɪ̯/, though the overall small number of phonemes remains unchanged. EDIT 2: Qualified claim that some dialects don't contrast /ɛɪ̯/--/ɛɪ̯/ is found in a number of English loanwords in colloquial American Yiddish, even in dialects which don't contrast historical /ɛɪ̯/, ex. די יונײטעד סטעיטן di Yunayted Steytn, the United States.