There's an open PR to add 5.1 support, and it seems to just be a bit-for-bit identical copy of the 5.0 code which Vagrant inexplicably requires a separate driver for: https://github.com/mitchellh/vagrant/pull/7574
All the heavy-duty OOP-first power of Ruby at their fingertips, and people still just copy-and-paste massive files to change a version string...
I only looked very briefly, but yes in this case 5.1 driver hasn't made any changes other than the version. But doing a quick vimdiff with a few of the other drivers show that the files have seen some substantial changes over time, and in some cases the small differences between two versions of a driver are to work around bugs, whether new or newly identified.