based on my previous acer experience, vista should be fine.
my 4400mah acer battery lasts 2 hrs for vista, 2 hours for ubuntu as well.
since MBP's battery is bigger, it should be definitely above 2 hours, if everything is right.
however, to take advantage of all the system power conserving tech (such as cpu clock slowing down, we all know in MBP, most of the time cpu is running at half of top clock speed), proper drivers need to be installed.
Im not really sure your situation tho, I can only say for the hardware and battery, vista should definitely last more than 2 hours.
edit: slashdot has this article
http://hardware.slashdot.org/article.pl?sid=07/05/04/1754220&from=rss
some comments indicate apple's driver doesn't offer step speed of cpu for vista, thus the cpu is running at full speed all the time, which is obviously a big power drainer.
maybe thats why vista runs on Mac faster? lol
🙂