Start Reaper and click "File" and then "New Project." Drag and drop the audio file containing the vocals you want to chart into Reaper's Edit Area. A new track containing your vocals will appear. Leave this track as it is for now.
Create a second track underneath the first by double-clicking within the track header area. Turn this track into a "MIDI out" track by right-clicking the "Record Mode" button and select "Record: output" then "Record: output (MIDI)" from the contextual menus. Click the "Record Arm/Disarm" button to record-arm the "MIDI out" track.
Click on the vocal track's "FX" button. A plugin selection dialogue appears. Scroll through the list to find the plugin named "VST: ReaTune (Cockos)" and double-click it. The ReaTune interface appears. ReaTune is a built-in Reaper plugin for analyzing and correcting pitch in real time. Ensure that the parameter labelled "Send MIDI events when pitch changes" is turned on and that the "Non-live input" option is selected.
Click on the "MIDI out" track's "FX" button and double-click on the plugin named "JS: IX/MIDI_Tool II" to insert it on this track. The MIDI_Tool II interface will appear. Leave the default settings for this plugin the same. This plugin receives the MIDI data generated by ReaTune as it analyzes your vocal track and use it create MIDI data that mirrors the pitch and timing of the vocals as closely as possible.
Click on "View" in the menu bar and select "Routing Matrix." The grid, or "matrix," representing the possible routing options between tracks will appear in the lower left. Find the row labelled "1," which represents the vocal track's send options, and the column labelled "2," which represents the "MIDI out" track's receive options.
Click on the box where the two parameters meet in the grid. A symbol appears in the box indicating that MIDI data sent from the vocal track will be received and transcribed by the "MIDI out" track.
Press "Record." Watch the ReaTune interface to see the audio data from the vocal track turned into editable MIDI data in real time. This data is sent to the "MIDI out" track, which transcribes as standard MIDI information in the Edit Area. Press "Stop" once the vocal track has completed. A new MIDI region appears on the "MIDI out" track containing the transcribed data.
Right-click on the new MIDI region and select "Open items in editor" then "Open items with built-in editor." Reaper's piano roll screen will appear and display the newly-written MIDI data as editable piano roll information.
Mute the original vocal track and click the "AR" button on the "MIDI out" track to record-disarm it. This will prevent inadvertent changes to the "MIDI out" track as you edit the transcribed MIDI data.
Press "Play." Listen to the transcribed MIDI data. It is likely that it will contain a number of wrong notes as a result of subtle pitch and timing variations in the original vocal track. Remove unnecessary notes by clicking them and pressing "Delete." Use Reaper's standard MIDI editing functions -- removing, lengthening and shortening notes, for example -- to edit the transcribed data until it resembles the vocal track in pitch and timing as closely as possible. You are now ready to export the region for conversion and import into the "Rock Band" game of your choice.