My Langenscheidt explains that this is a "Dankesformel", so would suggest "May God reward you"

"Vergelt's Gott" is normally used as a form of "thank you" — in the sense of "may God reward you for what you have done for me."

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Note added at 27 mins (2004-04-05 13:15:39 GMT)
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\"God bless you\" in the sense of \"thank you\' is also possible, of course — as is \"bless your heart.\" (These expressions may sound quaint to you, but the German term is a bit quaint as well.)

'God bless you' is used by many UK Engish speakers a formal and sincere expression of thanks (unless it's used ironically, of course). It is quite often used spontaneously, and although formal, it can also be used among good acquaintances and friends. However, it would be understood rather more literally by most US English speakers, and not necessarily as an expression of thanks. In the US it means something more like 'you're a (really) good person for doing that'. That makes it more an expression of appreciation than an expression of thanks. In the US it is most often used by truly religious persons or by persons who wish to (more or less ostentateously) give the impression of being religious.

It's an old Bavarian expression and merely means "thank you."