I don't necessarily recommend it, but I own a Vitus 979 and did this about 10 years ago so I could upgrade to a 9 speed cassette and STI shifters. I haven't had any problems with it. But I haven't put a lot of miles on it since the change and I'm under 150 lbs. Some of those miles have been on dirt roads.
I stripped the frame about a year ago and the dropouts measure about 128.5 empty.
The idea about trying to narrow the hub won't work because 1) you need the space to fit all the sprockets in the cassette 2) if you try to make space only on the left side your wheel alignment will be whacky 3) most modern hubs don't have spacers or cone washers that can be deleted from the assembly.
One reason, I haven't ridden it much since is because afterwards I never felt totally confident about the integrity of the frame. Also, once I was able to afford a new bike I enjoyed riding the new one more. I think you are better off keeping your Vitus as a classic and saving a up to buy a new bike with modern components.
If you're still considering it, remember that my experiment of one doesn't mean you'll have the same success.