Sforno on Genesis 33:18/ספורנו על בראשית ל״ג:י״ח

1ויבא יעקב שלם עיר שכם, he reached the land of Israel unharmed. This was the fulfillment of the condition he had set in his vow in 28,21 ושבתי בשלום אל בית אבי, “and I will return in peace to the house of my father.” He did not wait until he had physically returned to the house of his father.

2בבאו מפדן ארם, while he was still on the way to his father’s house. He began to fulfill the terms of his vow by building an altar This was in accordance with his promises והיה ה' לי לאלוקים. (the last words of his vow).