wav files do not contain any lyric or note information, just the audio signal to be sent to your speakers.
There is an upcoming feature for SynthV to make it possible to import a wav file and have the software infer the project data based on it, but this will just try to transcribe a project file based on your audio, it won’t have any insight into what the original lyrics were.
Could you cut the Reaper project to remove the SynthV portion, keeping only the small mp3 bit, import that into your SynthV project as an audio track and assemble the backing there, vocal changes are then possible and the final version (whole project with other tracks muted) could then be re-exported as a new wav file?