Can Melodyne editor or other tools resync vocals to new tempos, say from 100 to 120?

Never really use M. editor as of yet but want to slow music down enough to do a few toungue twisting raps then speed the song back up and have it all stay in time...and properly pitched.
 
My daughter often does this to songs to create nightcore vids  but it messes with the pitch, which of course, is the point of their creations...speed it all up...
 
Me?  I just want to slow down some funky stuff to the point where I can rap/syncopate vocally a message then raise the tempo again but get the vocals to be in sync with the new tempo.
 
If melodyne ain't the tool, is there one?  Thanks all
 
 

I think that the tool you want for this is Audio Snap
 
Start but exporting a mix that will be the backing track to the vocal you want to do.
 
Create a new project and import the audio.
 
Set the tempo of the project to be the same as your original project.
 
Now on the audio track enable the audio-transients view. then open the AudioSnap window and select "clip Follows project".
 
Change the tempo at the beginning of the song and listen to the playback to verify that it is slow enough for you.
 
Once you find the tempo you want create a new audio track and record your vocals.
 
Once you are satisfied with the vocal track select the audio-transients view for the vocal and open the AudioSnap window and enable the same "Clip Follows Project" option for the vocal.
 
Now set the tempo back to your original tempo.
 
you will want to try different render mode options for the offline render in the audiosnap window for the vocal track ( I suggest radius solo vocal) and export just the vocal track.
 
Once you are happy with the result import your new vocal track into your original project and continue mixing or whatever.
 

CTRL and dragging the right edge is simplest. If recording to a backing track, I would first copy to a new track, then trim that copy to an exact bar count. To slow down from 120->100, ctrl-drag to show 120% at the top right of the clip. Solo this and record vocals on a new audio track, and be sure the length of the vocal track and backing track are identical. Finally, ctrl-drag the right edge of the vocal track back to the exact bar count from the first step (should be ~83% in the upper right), bounce to clip, and check the algorithm as Sanderxpander mentioned.

You can use Melodyne if you want. Choose the tool for moving blobs. Set the time base to 1/32 so you can drag notes more finely.

Sanderxpander
Record your clip at the desired tempo.
Cut it so that it's an exact number of bars (let's say 8).
Change tempo. Watch as the clip now occupies a different number of bars. Grab the edge, hold CTRL and drag it until it's exactly 8 bars again. It should already be working at this point, but you can bounce to clip for a better effect. Using A you can check in the AudioSnap panel that you have the correct algorithm selected (bottom right).

OK, couldn't resist... that seemed so easy... and it was!  Awesome!  I'm giddy as I head off to do some gardening!