June 29 (Reuters) - Shares of Dingdong (DDL.N), backed by SoftBank Vision Fund II, opened 19% above their offer price in their market debut in the United States on Tuesday, giving the Chinese grocery app a valuation of $6.6 billion. Stock opened at $28 per share, up from the initial public offering price of $23.50 per share. (Corrects to say valuation of $6.6 bln, not $7 bln, in headline, paragraph 1)