Long Nguyen, a Vietnamese immigrant living in Nashville, first tasted Manischewitz at Thanksgiving last year. A relative opened a bottle of the sweet, Concord grape wine to accompany the family dinner of roast duck, baguettes and noodles.

Mr. Nguyen, a 45-year-old electrician, was delighted. “I asked her for another glass after we were done eating,” he says. Hooked, he bought bottles to give to his friends and relatives as Christmas gifts.

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