Two professionals have provided vastly different approaches to the repair of the damaged subfloor area (9 inches x 9 inches) next to my bathtub. This is in order to replace the vinyl flooring. The visible joist about 9" from the tub is not damaged. The adjacent joist is under the tub. One estimate involved cutting out the damaged subfloor (with a circular saw) after removing the entire bathrooms underlayment, then replacing with a new piece of subfloor prior to installling new underlayment and vinyl sheet flooring. Second recommendation - Simply pour self leveling compound in the damaged area, which will harden like cement, then put new underlayment and vinly flooring over the entire floor area. Which is better? BIG price difference.

Dito. You need to address the problem and not cover it up. It will peak its ugly head again if you don't take care of it the first time.