Hello and welcome!
The devel/git port does not have an architecture restriction. But it will not build on Vax if any of its build-dependencies are not available on the platform.
Note that there is a separate list of run-dependencies that may also have architecture restrictions. This is not related to your particular problem, but if you download or build a package which you can not install, this might be the reason.
You can dig through the port Makefiles manually, or just attempt a build. The error messages will explain why devel/git cannot be built on the Vax architecture.
If you are unfamiliar with building packages from ports, FAQ 15 is where to go to get started.