Are you saying there isn't a choice about the envelope curve? (I've not installed Vegas 4 yet - I'm waiting to see what the forum users have to report about the update) ;-)
Yes, there are choices of curve types. You can set the default curve under the preferences section as I described above, and then you can also change the type by right clicking on the evelope point and changing it with the drop down menu. There's actually 6 different types of fades to choose from.
There is one slight oversight, or maybe some may view this as a feature. When you change the default fade type in the preferences menu, it does not change the default setting for a previous track. For example if you have "smooth fade" originally selected in the defaults and you have an evelope put on that track with the "smooth fade" defaults, changing the default fade to "linear" does not change subsequent points you add on that track to the now new default of "linear". Instead it keeps the original default of "smooth fade" throughout the track. If you add a new track now, with the "linear" default set, and then add points, it will default to the current "linear" fade type.
So, you may see this as a problem, or you may think this is good, because it allows you to set default fade types per each track if you wish. In either case, it's pretty easy to right click on the point and select the fade type if you wish to mix and match fade types throughout a track.
I was thinking, when you right click on the track you get the "set Default track properties". Maybe putting, a check box in there for Default fade type under the "set Default track properties".
Lol!!! That's exactly what I was thinking, but I was also thinking "SonicJEV" must be a software engineer. I work with a lot of software engineers and I find they start to talk in very short incomplete sentences and IF_THEN_ELSE statements when explaining something.