The Vaio Pro 11 is not just "slightly" lighter than the MBA 11:
- The Pro 13, with touchscreen, is virtually the same weight as the MBA 11.
- The Pro 11, with touchscreen, is IIRC about 10 ounces lighter than the MBA 11.
- The Pro 11, if you can get it without touch, is about another 3 ounces lighter.
That for me is by far the biggest selling point of the Pro 11 - a fairly capable, usable laptop that's possibly lighter than a new iPad Air & an Apple BTKB combined (older iPads are of course heavier), but with the full "desktop" OS can do so much more than an iOS/Android tablet.
If you're in a position where you don't "get"/don't care about such a weight gap, then you're probably not the crowd Sony's targeting, and you can check out plenty of other options (e.g. Lenovo always likes to build heavier systems than everyone else, complete with the benefit that brings.)
Few people realize that across the board, Apple's the indisputable king of battery life in consumer electronics right now, in a world where people no longer get thrilled at such claims. This is also very much the case for the MBA - great battery life out of technically the same config as everyone else. Sony and other Japanese manufacturers, in spite of having 15+ years of experience squeezing battery life out of the parts of subnotebooks they create, are ultimately at the mercy of Intel on this matter.
So I guess the decision is very much about battery life versus weight. Sony sells an external battery for both Pros that would bump their weight to MBA-levels, but that's an extra investment some might find absurd (i.e. why did I want such a svelte laptop in the first place, anyway?).
I've seen the Vaio Pro 11 and 13 in person, and they're by far Sony's best laptop effort in years. Last few years' models have clearly been built to a cost and often look/feel unreasonably cheap for their price points, but this is not the case for the Pro. If you're at all infatuated with what Sony brings to the table, I think this would be a good year to take the plunge.