I've also put the sample on one track and created a send to another track with the plugin loaded into it. Always check the phase by flipping the polarity. Is there anything similar for Reaper that has a central channel as well as right and left (more sliders), as per the plugin used in the tutorial? The JS plugin already mentioned has a mode only for left or right channel (or both) which didn't work. In this short FL Studio 20 tutorial, I will show you a few tips and tricks from my experience dealing.

The guy in the video uses something called Fruity Stereo Shaper to create an 'inverted phase cross mix between left and right channels' so it ends up taking out the vocals - although I need the reverse. I watched this video and although in the example shown the guy is using FL Studio, surely there must be a similar plugin for Reaper that would do this? Having searched around I found the closest plugin is 'JS IX/StereoPhaseInverter', however while it inverts left, right or both that pretty much silences either my left or right speaker or both. Usually I think phase cancellation is used to achieve the opposite, to remove vocals from the track as far as that's possible - obviously it'll never be a perfect result - but that's beside the point. In my case I'd like to keep the vocals and remove the music as much as possible. I'm trying to achieve phase cancellation on a sample with a vocal in Reaper.

Reverse polarity fl studio