From their website:
"You can use NCS music in most video types like reviews/unboxing, pranks/comedy/fails, beauty, fashion, fitness, lifestyle, gaming, sports, pop culture, DIY, design, compilations, and more. We currently do not allow content which is a music upload (music visualiser, music player, lyric video) and does not fall into the above catagories. Meaning, you should only use our music in the above types of content, not simple reuploads of music."
Now, obviously what I want does not fit into the stated categories, BUT it's not a video upload. It's also not a simple reupload, visualiser etc. So can anyone here help me with this, maybe has some past experiences? Thanks a lot.
Their list of use cases seems like it's for unaltered (non-derivative) uses.
If they don't say you can make derivative works, that means you can't make derivative works. (Which is what it would be if you used extracted vocals.)
But you should try contacting them to confirm, really.