Given Darth Vader's portrayal, even within the novel of having a disdain for anything less than direct confrontation, it seems odd that he would involve himself in biowarfare. But it should be noted that the only person saying that it was Vader's fault was Xizor, who could have fallen victim to confirmation bias by already holding Vader responsible for his family's deaths.
Or alternately, Vader was framed for the incident to create hostility between the two. Now who do we know that has a known fondness for making powerful figures within the Empire fight each other and is even explicitly shown in the book deliberately stoking that particular rivalry?
