Sumelic is right. I just want to add a few things. First, the expression in Deo vivas and related (e.g. vivatis in Christo, "may you live in Christ") seem to be of common use among Christians of the period. For instance, it's also found in a ring saying "Antoni vivas in Deo" (it appears reversed):
Other examples are cited here and here.
Interestingly enough, it seems a ring containing a similar expression (the so called "Ring of Silvianus", with the inscription SENICIANE VIVAS IIN DE) was an inspiration for J.R.R. Tolkien in his creation of the Middle Earth mythology.
Finally, regarding the slab in question, this site provides further speculation about the context and meaning of it. It does not mention anything about who Severa might have been though. Who knows, maybe it was Marcia Otacilia Severa, a 3rd century Empress of Rome, with the article suggests (without providing sources), could have been a Christian.