Or HaChaim on Genesis 27:28/אור החיים על בראשית כ״ז:כ״ח

1 ויתן לך אלוקים מטל השמים, "May the Lord give you of the dew of the heavens, etc." The resason this verse commences with the conjunctive letter ו is because it continues the theme begun in verse 27, i.e. ויברכהו, "He blessed him." Those blessings had not been detailed because the Torah merely told us that just as the key to blessings had first been entrusted to Abraham and subsequently by Abraham to Isaac, Isaac now handed this key to Jacob. (based on Bereshit Rabbah 39,11). Accordingly the word ויברכהו means that Isaac transferred this key to blessings to Jacob.

2It is also possible that Isaac divided the blessings into both spiritual and material ones. The spiritual blessings are referred to by the word ויברכהו, whereas the material blessings are introduced by verse 28. The letter ו then introduces the material blessings as something additional to the spiritual blessings Isaac had already bestowed upon Jacob.

3Aternately, the mention of the word האלוקים, the attribute of Justice, is to show that whereas up to then Isaac had invoked only the attribute of Mercy, he now also invoked the attribute of Justice, asking it to agree to the blessing he bestowed on his son.