Hardibacker is not waterproof nor is it a good underlayment for vinyl tile or sheet flooring.

The bbest water proof flooring will be sheet vinyl. There are no seams or grout lines. Sheet vinyl(and tiles) need a smooth surface on the underlayment. You can do that fairly inexpensively by installing cheap 3/4" thick ply on the joists. Then add a layer of rosin paper or 15 pound builders felt(roofing underlayment) That layer eliminates any squeaks from plywood rubbing against plywood.

Then install a good grade of AC plywood with the finely sanded side up. Use a seam filler to fill the seams and apply the vinyl flooring adhesive directly on the plywood and then the sheet flooring.

Sheet flooring can be tricky to install as a first time DIY project, so having pro's do that part might be best.

If you do want to try it yourself, use heavy brown paper to make a template of the entire floor, lay that over the sheet vinyl and cut. Be careful not to tear the vinyl as it is installed on the floor--dry fitting it---NO adhesive. Once the vinyl fits, fold back an area, apply adhesive, and roll the vimyl back over the fresh adhesive.

Then do more of the same until all the vinyl is adhered.

You will need a 100 or 200 pound flooring roller(rental mplaces have them) to rollm the entire floor to securely affix the vinyl to the adhesive.

If DebraV wants everything to be level with the oak outside, the thickness will have to be brought up to equal. Probably:
3/4" CDX sheathing, glued and screwed down, then 1/2" BC plywood stapled down, then the floor installers will use their own underlayment of around 1/4" under the vinyl, which should make the bathroom level or a little better. If DebraV was willing to allow the floor to be a little lower, they could use 3/8" BC instead of the 1/2".
Casey

Thank you both for your detailed replies. We put Trafficmaster Allure flooting vinyl tile in DH bath a few years ago, we are very happy with it. DH said the same as you both did 3/4 CDX. I have read so many posts about using T&G OSB, I just wanted to make sure we were going the right way. Thanks for reminding us to use the paper between the ply and next layer. I have a water leak fobia now, so I had DH flip the hardibacker over to the smooth side under the vinyl in his bath, thinking if we did have a bad leak the hardibacker would absorb the water without seperating like plywood will. Thanks again

Hardi backer will delaminate just like plywood if there is enough water. Besides, it is NOT a good substrate for vinyl of any kind.

If you wanted to match up the floors, like sombreuil was talking about, make sure to remember that most the plywood is 23/32. So if you have two layers of of metric plywood then it will not match your old flooring. I bet you both floors are a true 3/4. I am just fixing sections of my floor from dry rot and termites all around the house. I have the same floor. I had to buy more 3/4 inch pine boards and bought some utility grade oak flooring from lumber liquidators. I will be covering it all with tongue and groove plywood then adding my finished flooring.

Oh, I forgot. If you are that paranoid about water then check out this company. Schluter. The products will turn your bathroom into a tub itself.lol. I want to use the ditra product when I get to my bathroom project. Home depot sells some of the items. I think they are pretty spendy.

Here is a link that might be useful: Schluter Company