for RTMP stream playback is OBS much better than VLC .. And have builtin support for output to Decklink ..
But for RTMP stream playback im using small raspberry pi .. Its cheap, small , works great .. If U need SDI , then u can add small HDMI to SDI converter (powered from RPI) .. Its all together cheaper then single Decklink card
.. On RPI is raspbian with OMXplayer (commandline utility) running in infinity loop script ..
I was builded stream decoder for friend .. 2 decklink quad .. 16 SDI outputs for decoding IP streams ..