It is highly likely that the only defence possible is a mental health one considering the crime and it's subsequent actions.. the bit I doubt, with respect, is that his defence has now obtained the necessary medical materiel from the Netherlands.. it is precisely because they haven't, that the trial has again been moved forward.
The difficulty with any opinion in regard to Vincents mental health that a Dutch Doctor can attest to, is that it must be subjected to cross examination, and inevitably , put before a jury. It is highly unlikely that an Australian court will accept any opinion in regard to Vincent's health at 9 years of age, when it is supposed he attended a medical officer in regard to his behavioural problems, without an opposing point of view forwarded by the prosecution. Or more likely, a motion to dismiss due to irrelevancy. Unless Vincent was under constant and regular medical observation and rehabilitation, plus medical treatment throughout his sojourn in Holland, and then passed onto an Australian doctor for continuing treatment here in AU , of course. ( There is no reason this wouldn't be done, under the national health regime in Holland and the Health Scheme in Australia. Vincent's treatment would be covered entirely in both countries ) But there has been no mention of Vincent seeking, or receiving treatment for any mental disorder here in AU. Which is not to say there hasn't been, merely to point out no one has mentioned it, and certainly not his solicitor at his first appearance before the court in Griffith , NSW.
It seems, therefore, that a lot of reliance by the defence is being placed on this 'letter ' apparently of a medical nature, that has taken considerable time to even begin it's long journey to Australia ( in the year 2016) ... According to the defence, the contents of this letter have a major bearing , even the only bearing , on what Vincent will plead to. Guilty or Not guilty of the murder. Obviously, the letter will be about mitigating factors that intend to point to the inability of Vincent to understand the rightness or wrongness of his actions. . no more, and no less. That is all it comes down to. He can be afflicted with raging depression, or any sort of psychosis, but the bottom line will be could he tell the difference between right and wrong? .
The factors that point somewhat to Vincent's capacity to discern this is right there in his actions after the murder ( although all those web sites and enquiries are major stuff PRE the crime).. What motivated Vincent to take the body up to Cocoparra ?.. had to be a distinct realisation of the wrongness of his situation. It took a lot of hard work and time to attempt to burn Stephanies body, and all in the shadow of being open to discovery , then having to get back home, etc.. all these actions, in the psychological spectrum confirm that Vincent knew well that he had to take action to hide his previous action. .
So I am not convinced that the 'letter from Holland' is going to go any distance in providing a basis for a plea of mental impairment.. It is possible, of course, but I see it as a highly unlikely conclusion to draw. His employer was satisfied with Vincent's work ethic and capacity and seemingly was unaware of any psychological illness in Vincent, and then there is the question to ask, if Vincent did have some impairment, that made him dangerous and unpredictable around females, why did his mother and older brother not speak up and prevent him from working in a rural school?