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.