First, I want to start by saying that yes, I am quite opposed to the lack of a true Vocal practice mode in Rock Band 2. It might seem hypocritical to denounce this and yet at the same time wonder if the Vocal section of Rock Band 2 will lack needed challenge. I'll touch on this later.
As of now, there are probably several dozen players cross-platform that have gotten 100% on everything but Next To You on Expert Vocals(on the main-disc). I've noticed in the weeks and months past that more and more people from other instruments have tried their hand at Vocals, making it the Flavor of the Month. Some have found that with time and practice, they too could reach the level of other top vocalists. The largest hinderance appears to be learning and dealing with the broken talkie parts of Rock Band 1.
So, if the new phoneme-detection system in Rock Band 2 will basically eliminate any challenge for these phrases, it's fair to say that Vocals will become noticeably easier and more accessible. However, even with broken talkies and a useless practice mode, many people still have found success in Vocals. I really believe at this point that if the top Vocalists now listen to the upcoming songs, we will have little trouble in getting 100% on virtually everything almost instantly. So, I have to ask, will Rock Band 2 Vocals be too easy?
I've listened to 73 of the non-bonus songs, and feel that only 5-7 or so will present any moderate challenge. Yes, there are a few difficult songs, but for the most part, the tracks seem to be on par or easier than those of Rock Band 1's main disc songs. That's somewhat concerning.
A fully-functional Practice mode would make a potentially easy Vocal section even easier, but omitting this function isn't the right way to add difficulty. Frankly, the entire section needs a difficulty bump in my opinion. Make Medium become Easy, Hard become Medium, Expert become Hard and then add another level of difficulty to replace Expert now. Is this too much? Perhaps, but I really think most of the top Vocalists would be excited to take on this challenge.
Apparently, HMX is doing some work with the pitch-detection system of Vocals. I can only hope that a window tightening is included.