That said, if your development is more server-side, or you otherwise don't depend on those sorts of debugging tools, the iPad could very well make a good development environment for you.
If you just want to use Vim, then an SSH client like Prompt or iSSH would work well, combined with a remote server somewhere (which you'll need anywhere for viewing files). If you want an option that doesn't require network access, there is an iOS port of Vim, but it's worth noting that the files are only accessible through iTunes (i.e. you can't upload them to an SFTP server or view them locally with Safari).
If you want a more native experience, Diet Coda is worth a look, as it integrates a lot of useful tools — SFTP, a full SSH client, code editor and browser. It's definitely on the pricier side of iOS apps, but I've read good things about it.
Textastic is another good editor, with some nice extensions to the software keyboard (useful for making changes in a pinch if you don't have a Bluetooth one with you). I'm not a big fan of its file access model however. There's not much concept of syncing — rather you download files from a source (SFTP, Dropbox or WebDAV), edit them locally, then re-upload. An option to edit directly into Dropbox or SFTP would be nice, but that's a bit of a personal preference.