(Copying the maintainers list because this discussion is probably of interest to other maintainers.)
You certainly can and should use GitHub for VXL development. That URL is the official home of VXL on GitHub. However, sourceforge is still the primary copy of the master branch.
The master branch on sourceforge is automatically pushed to master on GitHub to keep them in sync. To make your commits an official part of VXL, you (or another VXL maintainer) need to merge/rebase them into/onto master and push master to sourceforge. The GitHub master will update automatically.
That said, you can ONLY push the master branch to sourceforge and you need to be a VXL maintainer to do it. You can push whatever topic branches you want to Github, and Github has tools to help review these branches. Furthermore, non-maintainers can also push to Github and request that a maintainer review and merge their work. So GitHub should be used for collaborative work, code review, and to accept contributions by non-maintainers. Sourceforge should be used by maintainers to make the official changes to the master branch.
We never really established an exact workflow for how to best use GitHub in collaboration with sourceforge. I've been busy lately and find myself contributing to VXL less and less. So I haven't devoted the time to organize and write guidelines for an official workflow. I probably should do that at some point. For now, you can use GitHub however you see fit to collaborate on VXL work. At the end of the day, someone just needs to merge completed work into master and push to sourceforge to make it official.