Will the vinyl hurt my paint?
Probably not.
Since we do not know how your walls were prepped and painted in the first place, we can't guarantee that removing the vinyl lettering from the wall will not cause paint to peel at some point in the future. In most cases - no problem. When in doubt, ask for a sample and apply it to the wall in a location that won't show.

APPLYING LETTERS TO A BOARD OR SIGN - (click for instructions)

APPLYING LETTERS TO THE WALL
Even the walls in a new home can have dust or residue that will prevent the letters from sticking.   Always wipe down the wall area where you intend to apply your vinyl.  Especially in the bathroom or kitchen where soap or grease are in the air, a degreaser is a pretty good idea.

For smaller sized stickers: You can just peel the backing and stick the letters on wherever you want, but for most of us it’s a good idea to use masking tape to tape the lettering to the wall for placement before removing the paper backing. Make a light pencil mark at the top and bottom and each side so that you will know where to put the letters and so that they will be straight on the wall. Then remove the backing and align and apply the letters to the wall.

   For large wall pieces, tape your letters to the wall before removing the paper backing.  Carefully measure to make sure they are level... take a step back and look at the placement.   When everything is all centered and level and exactly where you want it, run a piece of masking tape along the top of the lettering strip, half on the strip, half on the wall, creating a hinge all along the top of the lettering strip.   

   CAREFULLY cut up between the words or letters at approximately 18 inch intervals... or smaller if you like.   Peel the paper backing away from the section... be sure your transfer tape and letters are still hinged to the wall at the top.   Now, press down from the top and adhere the letters to the wall, letting them fall naturally into place.   Apply each section in the same manner.   After all sections have been adhered, give it one more good rub with the squeegee and carefully pull the tape away.

TO REMOVE YOUR LETTERS FROM THE WALL:
If your lettering has been in place for several months (or years) use a handheld hairdryer to heat the letters and then using your fingernails, just peel them from the wall. There may be a bit of adhesive residue which can be removed with regular household cleaner - the Mr. Clean eraser thingy or Windex or something similar works just fine.