Create your text and use the Layer > Create Masks from Text. Twirl open the mask, add a keyframe. Copy the mask path from your vector object and select your text mask, move forward in time and paste the shape into the mask path.
Here's some links to some other discussions about this technique:
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.