Just so you understand, the exterior walls sit on the vinyl and plywood floor. So if the plywood is rotted, you will have to remove it and check the wall bottom plate. For a leak from below, the wall should be undamaged. But a roof, wall or window leak will likely rot wall studs if they are wood. Anyway, to be successful all of the damaged wood must be removed and then it will be very tricky to insert new plywood between the bottom of the wall and the floor joists. Then add extra framing under the new plywood at all edges to support it.
Also, the floor is bolted to the steel frame with bolts directly under the wall. About every 36 to 48 inches. Locations can be identified by the steel outriggers extending from the main frame rails and the end of each bolt will be visible. If the rotted plywood is in the area of one of the bolts, you will need to cut close to the bolt and chisel out the rest of the remaining plywood. Also, loosen the nut and notch the replacement plywood.