HKey - Gõ Tiếng Việt is a free Android app developed by VNApps that enables users to type in Vietnamese using the Telex method. The app is based on the Android 2.3 keyboard source code, but it does not use prediction, dictionaries, or word suggestions to optimize speed and reduce RAM and battery consumption while typing. The keyboard's background color changes based on the theme of the supporting app (for Android 4.4 and later). The app is designed to support older Android versions, including Android 2.3, and is optimized for devices with low RAM.

This app is perfect for those who need to type in Vietnamese quickly and efficiently without worrying about RAM or battery consumption. The app is easy to use, and the keyboard is responsive and accurate. However, the lack of prediction and word suggestion features might be a downside for some users who prefer those features. Overall, HKey - Gõ Tiếng Việt is a great app for anyone looking for a fast and reliable Vietnamese typing app.