> Marathon shoes are much closer to Vibrams than to regular running shoes.
No, they aren't. Track spikes maybe, marathon shoes definitely not.
Have a look at the Adidas Adizero Adios [1], a shoe that Kimetto ran in to set the world record. It has a heel of 26.7 mm, and a heel to toe drop of 9.3 mm, and is quite stiff. A Vibram has maybe 4 mm of rubber, no drop, and is very floppy.
Yes, they are less cushioned than the average running shoe, but they are very far removed from Vibrams. I say this as an amateur runner, but I regularly run 100+ mile weeks.
[1] http://www.runnersworld.com/shoe/adidas-adizero-adios-3-mens
In fact, running on Vibrams on sand or grass is very similar to using cleats on the track. I say this as a ex-competitive runner.