I think Kuzui and Vertigo are doing this to force Whedon to get involved. Kuzui and her husband own the rights to the 1992 film, however, the supporting characters in the TV show (like Willow, Spike, and Xander) are owned by 20th Century Fox Television. So technically, they can make a movie with Buffy in it, since they own the copyrights to the movie and consequently, the title. Fox and Whedon paid Kuzui in order to produce "Buffy" and the spinoff "Angel", that's why they're credited as executive producers in each episode of said shows. (Yet they weren't on set at all, nor did they do anything else.)
If Whedon does become involved with this reboot or reimagining, Fox Television would have to become involved in some capacity in order to let Whedon use some of the supporting characters. But I really thought Kuzui and Vertigo would've pursued this earlier, either in late November after Twilight became a hit, or that Fox would've commissioned a big-screen spinoff in early 2007 to head off the WGA strike later that year (like they did for X-Files 2 and Dragonball).
I guess the success of J.J. Abrams' Star Trek was the clincher for Kuzui and Vertigo to get involved.
