Since Yakov already made a Neder to sacrifice to Hashem in Beis-El (28:20-22 ), why did Hashem see fit to instruct him to do it?
Rashi: This was a warning to stop dilly-dallying and to go immediately to Beis-El in order to fulfill his Neder, 1 because it was due to his delaying the issue until now that the incident with Dinah had occurred. 2
The episode with Dinah was because he locked her in a box (refer to 32:23:1:1)! Chizkuni - delaying the vow was due to the Aveirah of withholding her from Esav. To do so, he went to Sukos, Shechem and Beis Kel. Hadar Zekenim citing a Midrash: Also Elifaz took his money, the angel harmed him and Rachel died due to delaying to fulfill his vow (perhaps the Torah is out of order, and Hashem said this after she died).
Why does the Torah write "v'Shev Sham!"
Ramban #1: In order to cleanse himself from the Avodah-Zarah (of Shechem) or from the Tum'as Meis 1 of the people he had slain.
Ramban #2 and Seforno: To clear one's mind of other thoughts, in order to cling fully to Hashem. 2
What was the purpose of the Mizbe'ach?
Ramban and Seforno: To fulfill the Neder that he had made on his way to Charan twenty-two years earlier. 1
It can also be compared to what the Gemara says (Berachos 54a) regarding the Berachah 'Baruch she'Asah li Nes ba'Makom ha'Zeh'.
רש"י: לפי שאחרת בדרך נענשת: מנין לדרוש כך?
גור אריה: "קום" מיותר לומר- למה אתה יושב ומאחר את הנדר.
רש"י: לפי שאחרת בדרך נענשת: לעיל פירש רש"י שיעקב נענש על שהסתיר את דינה מעשיו?
גור אריה: כיון שמנעה מעשיו היא נפלה ביד שכם- מידה כנגד מידה, אבל אפשר שהגזירה הייתה מתבטלת על ידי תפילה ותשובה, אך כשאדם מאחר את קיום נדרו פנקסיו מתבקשים [-מעשיו נבדקים].