Programmergeek.
I believe that volume is the only issue you have. I look at the sweetspot issue as a bonus and not a detraction from vandersteen speakers (I guess I am an optimist). I believe the pursuit of good audio is a selfish pursuit given the technology available today and I believe you sacrifice a lot in audio quality if you do not have a sweetspot in your HT.
You should know that I have a 7.1 Vandersteen HT system and I love it. No one has ever told me to turn up the volume, people have told me to turn it down. However, it is a very high end system (Pair of 5's 4 VSM 1 sigs, 2 2WQ's, VCC5 Center (for guests only) and 2 V2W LFE subs) and can play louder than most Vandersteen systems with less distortion (I never hear any, perhaps my ears are not trained).
You should ask SportyBry about his system and loudness as he has a much better frame of reference than I do until you get the Quatros, which may be the speaker above all others for HT. More importantly, you should go to your dealer and play the Vandersteens loud enough for you to make sure you are fine with the volume issue.
As for my guests, they are not sophisticated enough to know the difference between sitting in the sweetspot or not. They just tell me that the system is the best thing they ever heard, so I feel I am not taking anything away from them as they sit on the side

.
My personal feelings on loudness are that I hate loud mid and high frequency effects and I love low frequency effects. This makes me far from a purist in movies as I do not want to feel I am in the middle of a
real battle. It is fine for me to know that a battle is going on

. This is what I do, I turn down the volume in the movie (reference level is way to loud), and I compensate by turning up the gain on my LFE woofers. This is obviously my own preference and would make many movie purists drop dead of a heart attack but it works great for me and might be something worth looking into for you.
To sum up, get Vandersteens if:
1) You will not have lots of guests or you don't really care that much about them
2) You are aware and comfortable with the Volume limitations of the speakers. Subs do allow these speakers to play louder btw, by about 5db's.
Sorry for the long post and I hope this helps.
Edit--Sporty, Thiels are a mighty fine speaker and you may like them better than the Vandersteens after doing this, but put a stereo pair of 2WQ's on either the 2CE or 3A sigs (with either a home made or model 5 xover) and then tell me about the midrange of the Vandersteens compared to a similar priced Thiel.