I've mentioned recently on another thread that the commonality between Similarities and Vocabulary is that both consist of very limited language input (single words), while Comprehension consists of lengthier questions, requiring more listening comprehension. This means that Si and Vc are primarily measures of expressive language, while Co is a mixed measure of expressive and receptive language. Comprehension also relies on contextual learning, which is quite dependent on one's cultural and experiential setting.
BTW, were extended norms used on the other two VCI subtests, as both reached the maximum scaled score?
How old is your daughter? If she is young, she may not be having problems because the demands are so little at present. Her PSI is quite significant, as it is not only a relative weakness, but a normative weakness. In particular, the Coding score would be considered impaired. How are her fine motor and writing skills?
I would second the suggestion to pursue further testing, but--full disclosure--I am a school psych, so I have a little bias toward assessment.
