Shortlink to this page: https://goo.gl/YmKKcM
VPSKeys is a freeware input method editor developed and distributed by the Vietnamese Professionals Society (VPS). One of the first input method editors for Vietnamese, it allows users to add accent marks to Vietnamese text on computers running Microsoft Windows. The first version of VPSKeys, supporting Windows 3.1, was released in 1993. The most recent version is 4.3, released in October 2007.
You can download the legitimate version here.
Note that there are some malware-infected versions of this floating around. You can verify the legitimacy of this clean binary by calculating an MD5 hash of it and comparing it with the MD5 stated on the VPS website.
Minh T. Nguyen
(former member of VPS)