How can I fork the original repo when I've already forked a different fork

There's no GitHub way (small lie, see below), but there's also nothing to fear.

By definition, your fork of a fork is a fork of the original. When you open a pull request, you get the option to choose both the origin and the destination for your pull request. The choices available there obviously depend on the fork graph, but as long as there is a path in the graph between the 2 repositories, you should be safe. Also, since pull requests live on the website side, you don't even need to add a remote as long as you don't want to use it from git.

Now of course, you might want to reconsider your place in that graph, and make yourself a direct child of the real upstream, but that's mostly unrelated.

As said earlier there is actually a twisted way to have multiple forks, which is to create organizations and fork in them. That way you can "own" multiple repositories in the same graph. But there's really no need to go there.

Thanks to sigma's answer, I saw that not only is the upstream repo available when I go to do a pull request on the jockm/vert.x repo, but all other forks of the upstream repo are as well. So what I ended up doing was:

1. Deleting my fork of jockm/vert.x and instead forking vert-x/vert.x, since mostly I want to work within the main upstream repo, not jockm's version.
2. Creating a branch for the commit I wanted to send to jockm, and a separate branch for the commit I wanted to send to vert-x.
3. Making the relevant changes to each branch.
4. Sending pull requests for each branch to the relevant repos, since the jockm/vert.x repo is listed as a possible target for the request (along with about 200 other forks).

I used separate branches (basically topic branches) so that those commits would remain the only thing in those pull requests, since subsequent commits on the same branch are automatically added to the pull request, and these changes needed to remain isolated until/unless merged.

It seems like the better option would be to create a branch on your fork, and create a pull request from that branch. You can use branches to "fork" your version