Flooring Tile - Porcelain tile in an indoor hot tub room

Hello! I had a 24' x 24' addition built onto my home last year and am getting ready to tile the entire floor. I have a large hot tub in place right now - one in which can't be removed without taking it apart! The subfloor is strong and the hot tub is supported well with extra floor joists beneath it. I do have some questions before I start the tile work. (I plan on working around the tub as I don't have any intentions of ever moving it.) First, will I need to put down a vapor barrier and if so, can I use a paintable type on the subfloor plywood before the cement board goes down? Second, am I creating a problem for myself be working around the tub? I do plan on purchasing extra tiles just in case the tub is ever removed so there will not be a probem with tile match up. Thirdly, there is a main support beam beneath the floor joists that runs down the center of the room. The plywood was layed down in such a way that it created a seam over the beam that runs the entire length of it. Because of this there is a dip on each side of the beam. When I place a 3 foot level on the floor I can see a 1/8" to 1/4" gap, depending on the location along the beam length. Right now my plan is to place thinset down and fasten cement board so that it crosses the seam by an inch or 2 so I don't have a seam on seam thing going on. Then when I lay my tile I will back up about half the distance between the seam lines to start my first tile run, again to avoid any seams. Does this sound like it will work? Any other tips for this particular tile floor installation? I could place another layer of plywood down but would like to avoid that at all costs.Thanks for your help!! What a great site!