I got an update about a achievement today so the week for vbulletin must end on a friday.
The reason I'm saying this is because the activity chart is a weekly bar that has one figure for each week based on how many posts you made each day, if you post the quote set by the admins in the day you get 100% activity, although the figure is also influenced by previous weeks so if you were inactive for the previous week you might not get 100% activity but the most up to date week has the strongest influence on your activity.
You can google it and the vbulletin forums say this.
Edit:
http://www.vbulletin.org/forum/showthread.php?t=93831 Here is the link for how it works.