Working on a bathroom remodel project and found rot and water damage to the plywood subfloor around the shower area. The shower is 36" x 36", and the joists are 24" apart and his part of the subfloor actually spans from the bedroom through this bathroom and into the next room. Was thinking of cutting out damage, adding some perpendicular supports and laying down new 3/4". Would this be correct? We're putting down a new acrylic shower pan and will tile the rest of the floor. Would I need a vapor barrier with the new subfloor too?
You got the right idea, but I'd opt for adding a double 2X? between the existing joists at either end of the bathroom span and fill in with an extra joist making your spacing 12 inches on center instead of 24. Add a couple of perpendicular stiffeners, and you will have a good stable base for your new 3/4" sub-flooring. Tile needs a good solid base or your grout will crack every time.
The vapor barrier is not so important on the floor, however, if you are doing new walls, a properly installed vapor barrier is very important. There are several methods of adding a vapor barrier, depending on what types of tile backer you plan to use. Think of it as a system and get all the right layers on in the right order.