Project in Rec 709 becomes Rec 601 (NTSC) when uploaded to Vimeo.

I suggest you create a new Destination.

My old destination for vimeo says "Rec 709", but upon creating a new one in 10.3.4 the only option I get is "Automatic". It seems that at some point one could chose the color space but now we can't.

Final Cut Pro->Preferences->Destinations, then drag Vimeo from the area on the right to the list on the left. Fill in the details. Try exporting using this new destination instead of the old one.