Based on my own personal experience with the `new` VM, I am unable to leave VM early with less than 48hrs left on it. Granted 24 of those are standard DP. It gives me a nice error stating that "Not enough time remaining" or something similar, I dont recall the exact wordage..
However, you can activate VM anytime you want, but if you attacked someone within 24 hours, the VM will not start until those 24 hours elapse. But if he was put into DP right after he hit, the VM will start 24 hours after his last attack, the DP will be in place before that begins.
I don`t know that for sure, not withour BW clarifying how exactly that works, but that`s my simple brain trying to make sense of it... Kinda silly, but not sure if it`s actually a bug.