A summarized account of the Apocryphal work Tobit (Tuvia in Hebrew) is supposed to appear in Bereishit Rabbah. (Source: Wikipedia) I have read that the midrashic account is connected to Jacobs vow (Genesis 28:22). Does anyone know where in Midrash Rabbah this account is located? (I could not find it in parshat vayetzei.)
This appears to be a mistake on the part of Wikipedia. The reference seems to be to an Aramaic manuscript of Tobit published by Adolf Neubaur[i]. This 15th century manuscript apparently mentioned a "Midrash Rabba d'Rabba", as its source. Although the identity of this Midrash is not clear, see Neubaur's discussion there, it may be identified with B'reshot Rabbati, a medieval work based on the writings of R. Moshe HaDarshan. Regardless, the reference is not to B'reshit Rabba, or the rest of the Rabba series.
I don't think there is any such explicit ref. in the Midrash to Tobit, nor is there any mention of Tobit in the Bereishit Rabbati (ed. Albeck) in the section of Jacob's vow. Dubious "connections" can be drawn between MR (Gen. Ch. 70) where the Midrash relates the tithes that Jacob designated and Tobit (1:7-9) where he too designated tithes on his journeys to Jerusalem for the festivals.
Alternatively, Tobit is reported to have instructed his son Tobias to present generous alms giving to the angel who aided in returning them safely (Tobit beg. Ch. 12) and this too is construed as a parallel to Jacob's oath uttered during his importunate state.