Is the Godox XPro flash trigger compatible with the Hahnel Viper flash system?

No, it is not.

Godox uses their own proprietary flash communication protocol, just as Hahnel does.

There are some nameplates that are rebadged Godox products, such as Adorama's 'Flashpoint' brand in the U.S. or 'Pixapro' in Canada and the U.K., Quadralite in Poland, some (but not all) Neewer flashes, etc. Anything that is made by Godox and rebranded using one of these other nameplates is fully compatible with other Godox products, as well as other rebranded versions of those Godox products. But that's where compatibility ends.

Hahnel's Viper appears to be a closed system with its own protocol.

Hahnel also makes some Canon RT clone flashes that can be controlled by a Canon RT transmitter or (theoretically) RT clone from another third party maker.

There are other third party flash systems that are "clones" of major manufacturers' systems. Yongnuo, for example, makes some RT series flashes and triggers that are compatible with Canon's RT radio communication system for flashes (not to be confused with Canon's EX optical communication system). Yongnuo also has several different protocols of their own that are sometimes partially cross-compatible with their other protocols. Receivers in their YN622 system made since 2015, for example, can be set to receive signals from a YN560 system transmitter or master flash, but YN560 receivers can't be controlled with YN622 transmitters.