The official software development kit provided by MediaTek for compiling .VXP files.
For most users who just want to play a .jar game on a VXP-supported smartwatch or phone, rewriting code is not an option. The easiest solution is to install a Java emulator that runs natively as a .vxp application.
Programs like Java Launcher are explicitly built to take .jar files and wrap them into executable .vxp files.