"Normal Nodes" can use any pair of pins for serial communication (e.g. pin 6 and pin7 or pin 0 and 1)...
"Vector nodes" receive new payload from the PC using pins 6 and 7, and they send the payload to the normal nodes via pins 2 and 3.
Have a look at pins_arduino.c for the pin mapping...
Just a reminder: the attiny84 does not have a U(S)ART. Serial Communication is implemented in software (assembler) and I testet it up to 115000baud at 8 MHz (internal RC clock), but the default is 9600 baud. In order to change the baud rate, you must change the variable "b" in COMasm.S
just search for this line:
b =14 ;9600 bps @ 1 MHz crystal