Welcome, ardenwood! (And apologies, lepa, that no one responded to your original post.)
I very much hope the VMI did not cost as much as $1000 per administration, even in your pricey area! Depending on how many components were administered and how quickly the student worked, 30 minutes might be generous. But I'm glad it was helpful for your DC's evaluation.
There are other types of dysgraphia, some of which would be better served by other instruments or more comprehensive evaluations, such as by an occupational therapist (if the concern is mainly fine-motor), or in academics, in various aspects of written expression (various types of professionals can do this). Executive functions--which are often impacted in the diagnoses reported by the OP--are also a factor for both mechanics and expressive aspects of writing. For example, self-monitoring errors and dysregulated attention both can explain inconsistent use of punctuation and capitalization, as well as spelling errors. Planning/organizing weaknesses can generate the difference between strong brief writing products and weaker extended writing products. EF would be best evaluated by a psychologist. Communication disorders can also impact written expression, at the level of the actual language being generated, even before it reaches the page, and would best be evaluated by a speech language pathologist.
Bottom line, there are many possible causes of writing weaknesses, some of which are in the category of dysgraphia proper. Among those, there are multiple types of dysgraphia. Appropriate remediation depends on accurate identification of the cause of writing weakness--which also requires an appropriately comprehensive evaluation of all suspected areas of challenge. Sometimes you just happen to pick the right area on the first try, but not always. (Your evaluator probably identified it more quickly because they also had WISC data and the accompanying clinical observations.)