Willie O was 6'4" with long arms & huge hands and yet was a rarely a lineout option ; that was the job of Tim Gavin or Troy Coker etc.
Jumping at the back of the lineout is very different from jumping at the lock positions at spots 2 & 4. There isn't a ton of lifting at the back of the lineout. Throws are less accurate so its exponentially harder for lifters to lineup the jumper. You have flankers lifting rather than props (less strength, less care factor - see scrums, abandoning of), so your jumper needs an aptitude for jumping, timing & body control in the air. Dave Lyons was 6'3" but was a fat 120 kgs and couldn't jump over a 10 cent coin. He was no Cockbain or Elsom at the back, which is why our lineout in that era of 2 opensides was raped by Scotland and others.
Anyway, that digresses from my main point which is that I reckon Vaea really is a one position guy. He doesn't currently possess the ball winning skills and workrate for either flanker spot. That may detract from his appeal to a S14 side looking for someone who can cover 2 or even 3 spots.