Bad floating vinyl plank installation.How to fix half inch strip
You have learned the hard way to get references, and phone them.  You could probably take them to small claims court and win, but it would likely be more trouble than it's worth.  Short of that you can try:


Put your complaint in writing, with photographs.  Go see him, and say that you want this made right, at no additional charge.  Vinyl flooring locks easily enough that he should be able to relay the floor fairly quickly.
If you don't get satisfaction that way, put a complaint in with the BBB.  


Note:  There are dinks out there who, when called out on a bad job, will make sure that there are going to be further problems with the job later.

I am not a lawyer.  The advice above is worth what you paid for it. Maybe less.



Normal practice at a doorway is to put down a transition strip.  This is a piece of moulding with a cross section of a short very wide T for equal heights transition, or a ramp down one side for floors of different heights.   The transition overlaps the floating floor and allows it to move.  

Plank flooring is easy enough to do, and from your question you know something about it, you may find it easier to buy one more box, take up the floor and redo it yourself.