Siri responds to 'Natural' talking & questions, vlingo has very specific commands. So yes similar in function and practicality, but the Apple version is technically superior. I don't suppose it will be too long before Android has something as good, though in practical use I don't really see there's much advantage to Siri vs vlingo, other than showing off it's funny responses.
There's a working 'Iris' app in alpha stage on the market which was added pretty much the day after Siri was announced.