I'm not convinced that isn't a double standing in for Frodo in that particular shot, though I'm not entirely sure one way or the other. What I find fascinating about that scene is that there are a few frames of it where EW and VM are actually playing off of each other and are almost the same height. This is not a criticism, because I think they really wanted the actors to be able to react off of each other in that scene, so they cut as much as they could out of each frame so that it wouldn't be obvious that they were on the same level.
It works particularly well once Aragorn has knelt, because then they are looking eye-to-eye. They could then shoot that part of the scene with both of them standing so that they could actually play off each other. It makes the scene a bit more "real" for me, though throughout the movie the actors do a fabulous job of conveying the emotion of their conversations even while they're not looking at the actual actor who is playing the particular character they're supposed to be talking to. It's really wild when you think about all the things they had to adjust to in order to make the size difference convincing.