Yes, it really looks like memory leak.
Could you, please, provide more info: your OS version and screenshot of VOX Preferences to see configuration, we will try to reproduce this issue.
Also, please, try the following:
1. open VOX Preferences and uncheck top 3 checkboxes (Use keyboard media buttons, Use headphones controls, Use remote controls), this will force VOX Media Buttons process to quit.
2. Remove VOX Preferences, install it again and turn on checkboxes back, VOX Media Button process will be started.
This will insure that really the latest version of VOX Media Buttons is launched. Because specially in last version we've made some improvements to avoid memory leaks, so it's strange you face such problem in this version.