Cairo, July 15. An arrival from Vicksburg, Saturday evening, brings Col. John Riggin, of Grant's staff, bearer of difpatches to the government. News important. Port Hudson surrendered uuconditionally on the 8th. Our troops raarched in ard took possession on the 9th. Thoy captured the entire garrison, about 7,000 men, 35 field piecei', 25 siege guns, and about 10,000 stand of sniall arms. We have Major General Gardner, Brigadier General Beal, aud Colonels Stedman, Miles, Smith, and one or two others whose ñames are unknown.