Using Visual Studio as IDE for mbed developement
There are two things here, one is the compilation itself, and the other is the IDE and debugger. For the compilation, Visual Studio does not have a compiler for ARM Cortex-M, so you're probably best using ARM embedded gcc instead. mbed and mbed OS already support gcc very well. Using Visual Studio as an IDE over ARM embedded gcc is probably doable. First you'd need to make Visual Studio aware of the build system, which is provided by yotta in mbed OS. You may find valinor and pgen useful there. These don't currently support Visual Studio though, so you'd have to implement that in pgen first.

Finally, for debugging, Microsoft recently released MIEngine, an open source plugin that enables Visual Studio to interface to gdb, which should provide most of what you need to get debugging working.

(disclaimer: I work in the mbed team at ARM. If you're interesting in contributing Visual Studio support to mbed (which would be awesome), come and discus with us over at forums.mbed.com and we can provide guidance).