Here's why:
* Closed syllables have a vowel followed by a consonant.
* Open syllables have a vowel at the end of the syllable.
In "grapevine," the syllables are broken down as follows:
* gra - closed syllable
* pe - open syllable
* vine - closed syllable
Since "grapevine" has open syllables, it is not considered a closed syllable word.