Vodafone Germany hosts mail for the following domains: arcor.de, arcormail.de, germanynet.de, gno.de, mail.isis.de, nexgo.de, nvvonline.de, onlineclub.de, oteloonline.de, oteloweb.de, rpplus.de, vodafone.de, vodafonemail.de. (I don't know for sure if this list is comprehensive.)
The SPF record for these domains is listed in the "spf.vodafonemail.de" DNS record and includes these IP address ranges: 126.96.36.199/27 188.8.131.52/27 184.108.40.206/29 220.127.116.11/29 18.104.22.168/24.
If you manually whitelist mail from various IP addresses and want to whitelist the IPs that send mail on behalf of Vodafone Germany, you'll want to update your list to ensure you're whitelisting all of these IP ranges.
Senders, if you're looking to segment mail out for Vodafone Germany subscribers, the list of domains above could be quite useful -- for example, for creating an MTA macro to adjust SMTP connection limits.