To the OP: My kid looks like yours in a lot of ways, though his gap between VCI and PSI is a couple of points larger.
For writing (and all writing-intensive work, including in-class tests in history, English and science) typing and strategies on breaking the work into bits has been his savior. For research papers, he uses Noodle Tools. For essays, he takes care to outline first. He MUST do those things. Probably always. I was always able at his age to just let the writing flow. Not possible with him. He has no trouble sitting still to do the work (for hours if necessary) as long as he's not too tired or his allergies aren't too bad. We all can lose focus some days. Now if I were to put a pen or a pencil in his hand, nothing would get done. He almost failed a recent English test because he forgot to type his answers and his essay question was answered in TWO words. And he knew the answer-- he just couldn't get it out.
For math, the jury is still out. He's in 7th and taking algebra and does not like to show his work. However, I've watched him and talked to him about it extensively and I believe it's more of an issue with him of just being flabbergasted that someone would expect him to write.down.something.so.obvious. He seems to be able to do it. Just doesn't want to. But a couple of grade dings seem to be helping that.
I would be interested to know if there are any examples of students WITH that type of gap between VCI/PSI who do not have difficulties with writing.
I've always waffled about whether to try and get a formal diagnosis for my son. What we have works for now, but I worry about HS.
