The 61st-ranked Vakulenko, from Ukraine, won by scores of 7-6 (7-3), 6-3.
Clijsters, a former world No. 1 from Belgium who is retiring at season's end, beat Russian Svetlana Kuznetsova in last year's final.
Another former world No. 1, Venus Williams of the United States, topped seventh-seeded Russian Elena Dementieva, 6-1, 7-6 (7-4); and fourth-seeded Jelena Jankovic of Serbia beat Russia's Elena Vesnina, 6-2, 7-5.
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