I would hope they respond in a logrithmic fashion. This is how faders are designed. Look at the markings on any hardware console and you'll notice the markings aren't linear, they're logrithmic which is the closest approximation on how your ears work.
ahhhh....I got ya. What control surface are you using? I just noticed something similar to this and reported it using the Generic Controler driver. Vegas seems to be intermittantly missing midi CC messages for me. Sometimes it works just fine when I'm moving a fader rapidly. Then I slow it down and pause for a minute and it stops responding, until my hardware fader reaches the extreme -inf or +12dB position, then all of a sudden jumps to that position.
I use the STC-1000 by Mercurial (www.thinkmig.com) (hey you can see me on the team page if you wanna laugh a bit :) )
Works fine in other apps so I doubt the problem comes from the controller.
The way it reacts here is really in a logarythmic manner. The higher the fader is on the surface, the faster the fader will move in vegas. This is normal since as you said, the DB scale is logarythmic, but usually the controls are translated to linear because it's easier to choose a good setting...
All values are 'normalized" and we do it according to how we saw other devices - Mackie, JLCooper, etc. It will work with a generic CC msg - which is essentially all these boxed send out, so with out actually seeing the data sent by the STC-1000, I really can't comment.