Does the Prime Video app support the Chromecast protocol now? Or are you referring to a full screen cast from an Android device, that's just mirroring the device screen?

Newer Vizio TVs use what's essentially an embedded/modified Chromecast as their smart tv OS, which works really really well. But since Prime Video on iOS (last time I tried) doesn't support the Chromecast protocol, I can't watch anything from Prime Video on my TV.