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, Scott Clements, certify, based upon a review of the Quarterly Report on Form 10‑Q for VASCO Data Security International, Inc. for the quarter ended March 31, 2018, 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/ Scott Clements 

Scott Clements

Chief Executive Officer

May 9, 2018