It depends entirely what you're trying to analyze. If we go with your premise, then it's still an "unknown" because we don't know what a Gen 2 Volt will do in the market.Comparisons against the first gen Prius are sort of moot. The Prius really didn't take off till the far superior gen II was released. When people say "look how successful the Prius has been," they mean the design used from 2003 on, and you need to start your comparisons then.
Of course you can pick random moments in the history of sales and declare that is the "right" time for comparison. Everyone is free to read whatever they want into the numbers. That's why I linked to the Excel file. The only thing the graph is showing is how many Volts and Priuses sold in the first 22 months on sale in the USA car market. No other factors were introduced or considered.
By that simple measure, the Volt is doing fine.