I just did some tests with my copy of Vegas 8 on my laptop and it turns out you can import ac3 into Vegas, but there's a trick to it: you have to import the VOB files from the ripped DVD directly into Vegas, which will create a Video Track of the mpeg-2 video and Audio Tracks for any soundtracks present in the VOB file (so if you ripped a single 5.1 soundtrack, one Audio Track will appear - if you ripped three soundtracks , three different Audio Tracks will appear).
Now having said that, I still prefer using the BeSweet method to create the 5.1 mix, because importing ac3 using the method I just described results in only one Audio Track to edit from - the BeSweet method, I feel, gives far greater control over audio editing/mixing because it lets you work with 5 separate Tracks as opposed to 1.