The visitors kept the momentum with a second try to captain Ngarimu Simpkins with a deft in-field kick from Vorenasu, which was converted, to open a 15-0 led.
North Otago were next to strike through powerful centre Faaitu Tuamoheloa after sustained pressure eventually created the gap. First five-eighths Ben Patston was able to add the extras.
Ten minutes later North Otago's explosive wing Whiria Meltzer crossed the line after second five-eighths Luke Herden sucked in two defenders to feed his wing. Patston added the extras to ensure close the gap to 15-14 at the break.
North Otago were unable to carry their momentum after the break with blown opportunities or poor handling.
East Coast and North Otago took turns at taking the lead courtesy of the boots of Patston and East Coast first five-eighths Clayton Kiwara.
But it was East Coast who struck with the all-important try with 14 minutes remaining after fullback Vorenasu scampered down the left-hand touch. He then chipped past North Otago fullback Billy Guyton and displayed his electric pace again to win the race to the ball, putting East Coast out to a six-point lead.
East Coast then repeated their efforts when they had 13 men on the paddock, as they chewed up time on the clock with a bucket-load of possession to take themselves into the final next week.
Vorenasu was the stand-out performer of the match, having a hand in all of his team's tries while being solid on defence at the back.
Tuamoheloa shone in a poor North Otago side that showed less hunger to win than their opponents.
North Otago 17 (Faaitu Tuamoheloa, Whiria Melzer tries; Ben Patston 2 cons, pen) East Coast 23 (Sikeli Vorenasu 2, Ngarium Simpkins tries; Clayton Kiwara 2 pens, con). HT: 14-15.