im user of "Harmony Assistant" and "Virtual singer" from years. The virtual singer module is very usefull, for sketch things, not that you will replace a real singer. In spanish, they sound a little artificial, but maybe in english and french, they are better in that aspect.
Harmony assistant is a very powerfull software for notation and more, but is maybe not what you can easily connect for midi-edition of reaper. Yes for a final touch of a midi recording to make an score.
You can try them anyway, they are shareware and few limited to try.
BTW, here is a little video i made scrolling the score output of HarmonyAssistant, can give you an idea:
http://www.soundclick.com/bands/defa...?bandID=775578