It seems strange to me that you would get LPCM from a cable TV box. Been a while since I used one, but usually there is an audio setting to enable 5.1.That worked as far as the "routing".
I set ARC and EARC modes on the Vertex2 to Auto and turned off eArc on the TV.
However for the cable TV audio I get "L-PCM 48kHz 2.0ch 16bit", not 5.1. Trying to figure out why now.
I could understand getting LPCM from the Samsung TV ARC if the audio format was set to PCM instead of Auto or 5.1. Check if your Samsung has a 5.1 ARC audio format option:
Samsung... Menu > Sound > Expert Settings > Digital Output Audio Format