Vendoring works great for us, we keep everything in one big repo and it's open source. The problem is that the current tooling for managing this is horrible.

What is horrible about managing it? The biggest problem I encountered with vendoring is getting rid of git submodules.

We currently use godep, and it breaks very easily due to tags and differing architectures and OSes (among other things). In addition to being unreliable, it's pretty slow. I'm not aware of any other tools that do import rewriting+vendoring and work better than godep.

Oh, do you check in the rewritten packages into git?

Yeah, that's generally what "vendoring" means.