I want to give a quick overview of how I see the Yiddish dialects emerging from a Gothic system. Early Yiddish developed out of Gothic in parts of Ukraine. Until about 1200, Early Yiddish remained in eastern Europe. The spread of Yiddish into Central Europe began in the 1200's reaching Austria first. There speakers of Bavarian German learned Yiddish. The Yiddish they learned was essentially a Gothic system but in the course of acquiring it they incorporated many Bavarian features. For example, the vowels in the words inherited from Gothic were redistributed to approximate the Bavarian vowel distribution. The resulting Bavarianized Yiddish constituted the ancestral form of modern West Transcarpathian Yiddish.
This Bavarianized Yiddish spread into Germany where it was learned by speakers of Central German dialects who modified it by incorporating material from Central German. The German Jews of the Rhineland also incorporated their unique and very old Hebrew vocabulary. This was the origin of the family of West Yiddish dialects that came to be spoken in Germany, Alsace-Lorraine, Luxemburg, Holland and Switzerland.
In all these dialects the incorporated German material was modified owing to its integration into the basically Gothic system of Early Yiddish. The nature of these transformations is a rich source of information about the structure of Early Yiddish. This particularly valuable as we have no texts in Early Yiddish.
A migration of West Yiddish speakers into eastern Europe began in about 1400. There these West Yiddish speakers came into contact with resident speakers of Early Yiddish. The newcomers acquired the Early Yiddish system while adding West Yiddish elements. The new forms of Yiddish that developed out of this process were the ancestral East Yiddish dialects.
This process had interesting parallels to the earlier development of West Yiddish. In that case a vowel distribution approximating that of a German dialect replaced the Gothic vowel distribution of Early Yiddish. In this case, the Gothic vowel distribution was replaced by one from West Yiddish.
Just as the transformations undergone by German elements integrated into West Yiddish testify to the structure of Early Yiddish, the incorporation of West Yiddish elements into East Yiddish gives
additional testimony about Early Yiddish structure. In both cases the transformations reflect structural processes that Early Yiddish inherited from Gothic.