For purchasing top ups in Germany you may try
https://secure.verivox.de/cards/
No experience, however. This is the organization to which teltarif refers. You will need to register at the site. To be expected, however.
I have ALWAYS purchase tops up cards when in Germany. I started a SIMYO account for my wife via credit card while she was there. I will have to top that one up before it expires also via credit card. I can refill my D1 phones via the t-mo.de site and I may do that. They are currently offering a bonus to refill but I don't need to at this point. I would rather let the phone go into message time and then try EITHER my refill cards OR wait until May. Last May they also offered a bonus to top up via their site. The phone time will run out about mid-May 2006.
Vodafone DE has good network coverage in DE, but I have no personal experience with them. I am wondering WHY you would choose them? I chose T-MO hoping I would have some compatibility with my US T-MO phones and that is the case in certain phones.
Good luck.