This source code was provided by Vizio based on a written source code offer on 6-June-2016.

Covers MediaTek MT53xx/MT588x-series chips:

MediaTek MT5368 (Cortex-A9 single-core)
MediaTek MT5368_FPGA (ARM11)
MediaTek MT5396 (Cortex-A9 dual-core)
MediaTek MT5396_FPGA (ARM11)
MediaTek MT5369 (Cortex-A9 single-core)
MediaTek MT5389 (Arm11)
MediaTek MT5398 (Cobra Cortex-A9 dual-core)
MediaTek MT5880 (Viper Cortex-A9)
MediaTek MT5881 (Arm11)

Real-world devices known to have these chips:
Vizio E500i-B1 LED Smart TV
(Unknown models) / Hisense

Also see:
Rooting MediaTek Based Linux Smart TV / xda-developers
http://forum.xda-developers.com/android/software/rooting-mediatek-based-linux-smart-tv-t3150281

Linux/U-boot source for Vizio Smart TVs based on MediaTek MT53xx/MT588x
https://github.com/spartan263/vizio_oss