Looks neat, will give it a go. I think you may be in violation of the GPL for your vim plugin since you are creating a combined work but are not releasing the TabNine source code under GPLv3.
It looks like it is probably OK. The vim plugin it is based on seems to have already been designed to run using a client/server architecture. The plugin is the client, and it gets its completions from a server.
He just changed it so that it uses TabNine as that server.
Not cool in my book regardless of legality. Rebranding it to tabnine-vim alone is confusing, since none of the legwork for vim support belongs to TabNine. At the very least the original copyright notice should be left intact in the README (iiuc this is required by GPLv3).
It includes a copy of GPL. The README tells you what it applies to, and what it does not. It tells you where to find the original project it was forked from. And all the files that the TabNine guy did not write contain their original copyright notices from the YouCompleteMe authors.
https://github.com/zxqfl/tabnine-vim/blob/master/COPYING.txt