I posted a similar question over in the project Sputnik forum with some research I've done, you can read about my experience over there.

Am I waiting on type c support AND TB15 hardware drivers on linux before things will work -- or am I just waiting on the TB15 hardware drivers? Any clarification about exactly what I should be looking for to get the dock working would be appreciated. Thanks!

USB with the type-C connector already works on Linux. As you've noted, the USB-C to Ethernet adapter works, as does the VGA adapter. One of the problems is that the dock is not USB. It's Thunderbolt (a different protocol) that happens to use the type-C connector. Right now we are waiting on several things: Thunderbolt support in the Linux kernel (which is coming from Intel) and then Linux drivers for the individual ports within the dock itself.

Right now Linux can't even see the dock at all, because it doesn't have Thunderbolt support in the first place. Some of the very latest kernels (4.4.5 as of this writing) may support something, but I haven't been able to get my kernel past 4.3.3 without boot issues.