These trade paperbacks collect many of the sidestories found in the "Classic X-Men" line that reprinted the early part of Chris Claremont's run and filled out the pages with extra content. With more than a decade of perspective, it allowed Claremont - with occasional help from Jo Duffy and Ann Nocenti - to flesh out his early issues and enhance the original product. For instance, the attraction between Wolverine and Jean Grey is much more prevalent, Nightcrawler is a bit more of a swashbuckler from the get-go, Storm's love of plants is addressed more quickly and Colossus' skill as an artist is on display.
"Vignettes" answers a lot of questions. Why did the still-going-out-with-Scott-Summers Jean Grey leave the mansion after what happened on Krakoa in "Giant Size X-Men?" What happened between her and the Phoenix Force as the shuttle plummeted to its doom? How did Mesmero take down the X-Men, turning them into sideshow freaks for the notable circus issue? When did Colossus impregnate a woman in the Savage Land? (Having never seen these, Peter suddenly having a lovechild in an Annual really came out of left field the first time I read it.)
But there is a bit of a problem when it comes to presentation. "X-Men Vignettes" offers up its stories without context of what was happening in the original stories at the same time. Anyone who isn't a die-hard fan will easily become lost.
These stories are probably best experienced alongside the first Claremont X-Men omnibus (as it covers up to just before "The Dark Phoenix Saga.") Even then, I would suggest keeping up during a second reading for a more authentic experience.
It may be a moot point, though, since these books are extremely hard to find anyway.