The best Game Development alternative to Vulkan is Allegro, which is both free and Open Source. If that doesn't work for you, our users have ranked more than 10 alternatives to Vulkan, but unfortunately only two of them are Game Development Tools.
Allegro 4 and Allegro 5 are cross-platform, libraries mainly aimed at video game and multimedia programming. They handle common, low-level tasks such as creating windows, accepting user input, loading data, drawing images, playing sounds, etc.
Crystal Space is a mature, full-featured Software Development Kit (SDK) providing real-time 3D graphics for applications such as games and virtual reality. It is free (LGPL) and cross-platform (Windows, GNU/Linux, Mac OS X).
