i just purchased the ps vita mega pack bundle in the german amazon. can i redeem the code using my UK psn account?
When you buy something in one region, it depends on what the region your PSN ID is registered to, content from one region cannont be redeemed in another regions store.
Game cards will work on any PS Vita, but online content downloaded from PSN Store is region-coded. Any download codes you get are tied to a region and cannot be redeemed via another regions store.