That sort of curve is exactly what I was thinking of when I saw the VF5 sales (i.e. most traditional games see high initial sales and drop off significantly after the first couple weeks). The PS3 is a new platform and over the next 6 months won't see a ton of new titles, so new purchases will continue to breath life into VF5 as it will continue to stick out in the library.
For some perspective (thanks for the link Zeckensack!!):
VF4 1 week sales (02/02/02): 367,500 (All Time Japanese Total: 558k)
Similar time of year, of course by this time the PS2 had 7.9M units in Japan versus fewer than 700k PS3s in Japan right now. The VF 3TB numbers are probably the most similar (although going back that far probably opens far too many variables). Released around the DC launch in 1998 it was able to push out 110k units at launch in the first week, 367k total in liftetime in Japan.
It doesn't look very likely that VF5 is going to match the VF4 sales in Japan, which is unfortunate. Of course I think many gamers are looking for a new breath of life in the genre in general...
So based on your numbers, VF5 actually sold more units compared to PS3's user base (ca. 650K) than VF4 compared to PS2's user base (7.9 Mill) at the time. Approximately 7.6% vs. 4.6%
Of cours this is likely due to the less stellar PS3 game library right now. . .