I suspect that was their original intention (take Reaves + a pick for something), and then found out about the Brassard situation and the opportunity to mess with a division rival and give a bit of a thank you note to the team that basically gifted them Fleury.
I mean, I'm not going to weigh in on what they may have been thinking but I don't understand the Fleury angle. If there was any market for him he'd have been traded.
My understanding of the Fleury deal is that Rutherford spoke with Fleury very early on and told him Murray had to be their guy going forward (cap considerations, etc). He asked Fleury where he wanted to go and Fleury picked Vegas; essentially, they didn't try to trade him and reserved him all season for the expansion draft pick.
This
article hints at that, but I can't recall the original source I read this from. Why Vegas needs to feel particularly chummy about it, I am guessing is about adding an All Star to their franchise from day 1 helps with marketing and winning.