Interesting, I see it's not working out of the box with maniacbug-mighty-1284p 
__vector_20 is USART0_RX_vect. The 1284P defines the USART0 vectors, HardwareSerial picks them up. Shouldn't this also have the same problem on a Mega?
Looks like this library is demanding access to the USART0_RX_vect?