Although we've never met, may I say what a wonderful job you are doing with your son? [We have a brain-injured son, now grown, who also had/has a form of verbal apraxia, so I can guess at the road you've traveled. Because he is also retarded due to the injury, it is difficult to tell how much of his speech disabilities stem from his developmental challenges and how much from actual speech apraxia. Nevertheless, even when he has the vocabulary to express himself, it is generally difficult for him to retrieve the words he needs for placement in appropriate context.]
At any rate, you obviously are right on top of the situation, as indicated by your son's academic progress, likely achieved at least in part through your perseverance.
One might also note that the difference between his oral reading and comprehension is relatively small and not that unusual even for those without challenges. Again, my guess is that this is due to the individual attention that you have given to your son.
Now, I'm not familiar with the Vanderbilt program, nor how it might 'mesh' with CHC's curriculum. The following points, however, might help you make your decision about next year's placement.
1. Individual, personalized, one on one tutoring, administered by a person with intimate knowledge of the needs of the student, coupled with a personal interest in the child, invariably trumps group instruction.
2. CHC's program is academically sound, but also completely open to tailoring to the needs of the student. For example, sixth grade assignments include a fair amount of writing [primarily in social studies]; a student who is better able to express himself in writing might flourish with this approach. Yet, at the same time, assignments may be modified to fit the abilities and needs of the child, without loss of academic progress.
3. Homeschooling eliminates busing and drive time, wait time while instruction or correction is directed at other students, and almost immediately catches difficulties that the student might be having so they can be just as immediately ameliorated. Thus, in terms of catching up, homeschooling can be the perfect setting for the struggling child.
4. Any programs that you are currently using with success at school can probably be used at home with even greater success.
May the Holy Spirit grant you wisdom and insight as you make your decision.
God bless you!