USART RX Interrupt on an ARDUINO PRO MINI containing factory bootloader
Once the bootloader transfers control to the main code (sketch) it may as well not exist any more. It does not affect the way the sketch runs.




  I've written a simple echo program using RXC interrupt vector ...


Please post this code. For you to print "READY" you are (presumably) using the Serial port, in which case the HardwareSerial interrupt vectors are being used. I don't know how you managed to take over those vectors. Seeing your code might answer that.