The designers of La Sportiva Vertical K took some risks resulting in a unique shoe that works really well in some situations. They used a simple, mostly spandex upper and EVA foam midsole to create a light weight shoe that blends weather protection, breathability and a surprising amount of cushioning.
After testing on a number of runs, hikes and scrambles, I've concluded this is a close-to-ideal shoe for long, big vert days on good trails in inclement conditions. Its light weight (~ 7 oz) also makes it a perfect approach shoe for skiing/climbing days where shoes need to be carried while you're wearing ski boots or climbing shoes. Even heading uphill with skis on my back I found myself feeling light footed and running sections of trail in these shoes.
The Soft Sole
I was initially a little frustrated with the Sole of the Vertical K's as I found my feet getting beat up on long rocky sections of trail. The minimal drop (4mm) blown foam midsole offers a lot of cushioning but the lack of any sort of rock plate means there is very little protection from sharp under foot rocks.
On smoother trails, however, the mid sole provides a remarkable amount of comfort for the weight. I appreciated this pounding down 3000 feet of forest trail after an early season ski day. The flexibility of the midsole allows the foot to flex naturally and feel the trail which is a good thing until the trail gets too rough.
The sole itself is sticky rubber with small lugs and large grooves between the waves of the midsole. I found that it works well on dirt, smearing on slabby rock or wading through fresh snow. However the softness of the midsole limits the ability of the shoe to edge while scrambling up rocks or to kick steps in consolidated summer snow, limiting this shoe's usefulness off trail and above tree line. It is a running shoe optimized for use on terrain that can actually be run.
The Softshell Upper
The soft, spandex upper of the Vertical K is my favorite part of the shoe and I hope other designs copy it. It offers an excellent blend of weather resistance, breathability, comfort and durability.
Climbers and skiers long ago figured out that tightly woven stretchy "softshell" fabrics like schoeller and power shield held up best to aggressive use and maximized comfort in all but sustained day long downpours. I'm surprised it has taken shoe manufactures this long to catch on.
The spandex used in the Vertical K's offers similar comfort. I've been able to run through inches of sugary fresh snow and damp grass and keep dry feet. I've never liked goretex in footwear as I tend to sweat out so the amount of weather protection and breathability offered by these shoes was a pleasant surprise.
As with softshell garment fabrics, the upper also seems to improve durability in a surprising way. I typically wear out running shoe uppers in the flex zone, just behind the stiff toe bumper where the coarse mesh develops holes from repeated flexing concentrated into a small area of fabric. On the Vertical K this area is still going strong thanks to the easy flexing fabric and minimal toe bumper.
I also often get blisters or hotspots on my toes on long sustained descents as my toes are forced into contact with the shoe. The ergonomic shape and softness of the Vertical K upper have minimized this phenomena and my toes were happy even on a 6 mile descent.
I'm sure this upper won't hold up to repeated abrasion and the tiny toe bumper doesn't offer much protection from kicked rocks but I don't feel either of these things are particularly important in a running shoe. I try to place my feet between rocks when running or hiking and am willing to give up this sort of ruggedness for increased comfort and resistance to wear from repeated flexing.