The Vista is essentially the Vantage, with a slightly larger bass enclosure...and without an internal amp for the bass driver. It has the same size panel, etc. It does have a higher crossover than the vantage, I believe. It is in their newest lineup.
The clarity has a much smaller panel, and I believe a smaller bass driver?
If you have the choice between the two, I'd go with the Vista, no question. I have a friend with the clarity...and have auditioned the Vista. I plan on getting the Vista (and bi-amping it-a pretty cheap, but powerful SS amp on the bass driver, and AES Six Pacs on the panel)
In fact, in a small room, the Vista sounded better to me than the summits! *flame suit on!* Needless to say, the Summits were set up very poorly, in a tiny room, and I didn't have the chance to play with any of the bass settings or positioning. But for small spaces, and if you're not a bass head, the Vista is amazing.