Should be simple enough.
Unfortunately I could not get the importer to work in VSTS.
So here is what I did.
Made sure I had the latest master branch and all remote branches I wanted to include.
Change the origin.
Open up Powershell, or your favorite command line tool.
Run git remote set-url origin YOUR_VSTS_ORIGIN_URL. It is usually in the format of https://VSTS_INSTANCE_NAME.visual.
studio.com/REPO_NAME/_git/REPO_NAME
Push to origin.
Run git push -u origin --all.
That should do the trick!