EXHIBIT 32.1

CERTIFICATION OF CHIEF EXECUTIVE OFFICER

Pursuant to 18 U.S.C. Section 1350, as adopted pursuant to

Section 906 of the Sarbanes-Oxley Act of 2002

I, T. Kendall Hunt, certify, based upon a review of the Quarterly Report on Form 10-Q for VASCO Data Security International, Inc. for the quarter ended June 30, 2008, that to the best of my knowledge:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934, as amended; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Registrant.

/s/ T. Kendall Hunt

T. Kendall Hunt

Chief Executive Officer and Chairman of the Board of Directors

August 6, 2008