Vinyl look flooring over hardwood.

Going to install vinyl plank flooring over new concrete in mud room. I've read differing opinions on using a vapour barrier or not. The manufacturer website says its not required but I've read a few forums where they say if there is any possibility of moisture (I can't guarantee there is not) ,then a vapour barrier is required to prevent mold.

Hello, Well, I've made a mistake. I've learned that "stopping before the wall to do cuts later" is a bad, bad thing to do. Fortunately, it's only one small part of the larger job but I'm hoping I may be able to easily fix this (and my sanity). Context: My wife was laying planks today, and saving the end cuts for me. As this was our first time doing this, I figured..."Hey, that sounds good! Thanks for the help!". Well, cut to this evening and here we are. I've found that through slight variations in the tile and so forth, one little nanometer variance compounds itself as you go along the floor, and you find yourself with the last spot that isn't an even fit for the last tile. It'll fit initially, but 4" to 6" up, the width between the two adjoining planks narrows by, no joke, 1mm or so. So I can't lay the tile. How can I salvage / solve for this? Should I pull the adjoining planks and do it all together so I don't have to try to make a 1mm to 1 picometer gradual cut across 3 feet of tile? :( I'm lost here and welcome any feedback from those who are in the know. I've attached a picture.