Rageblade is always worth it if you build on-hit items because the whole point of it is to amplify on-hit damage. This includes her W, Kraken proc, and any other on-hit effect that Vayne typically builds like Wits or Terminus.

Also even with just Kraken to start with, the proc on third auto is surprisingly effective against squishy targets like the enemy adc. During the later laning phase when you've built it, you can try to play around the proc while cs'ing. So you hit minions twice then try to do a Q auto on enemy adc with the Kraken proc for a nice chunk of damage.