VVV is experienced as the ideal awaking sound, generated by a sounder, for rail road telegraphers in lonely stations, that were falled asleep, and gave them a short time to take their pencil and start copying the incoming message.
VVV was used as a sequence on a dead line, so that a recording papertape got started, before the actual message was sent.
Later on VVV was used as starting of machine code tapes for messages without meaning, only to transmit repeatedly the call sign and something like QRU? in order to keep the frequency occupied, and allow stations to keep tuned.
When you have paddles and don't know the speed. the keyer is adjusted for;the V is a character that tells you that.
VVV is a code that means "transmission in Morse code follows"
ZXZX as I may remember wrong: transmission in CCITT 5 RTTY follows