Of course I do, I think, it was setup for me when I first signed up with Vodafone. Who doesn't already have an account?
Surely this is a solved problem, and I must have missed a login link somewhere... but no, it turns out this is a common problem and there's a bunch of posts all over the Vodafone community forum and other forums about it.
The accepted solution appears to be some Vodafone community volunteer* manually deleting your existing account so that it can be re-created through the app.
Seriously? I don't understand how 'login' is a feature that was missed out of the app creation process...
* aka a Vodafone staff member that volunteers their time on the forum it appears, you guys are saints if that's actually the case and you do it on your own time