Almighty Zard
He has returned.
If the writers want Greninja to come back, then they'll write the vines are taken care of because he got every last one of them. It's simple. Just like how the Johto-Kanto monorail was up and running in BW despite being scheduled for completion in about a year in OS. And Ash is still 10 years old.
Or Lei is a 3-month year old baby despite Burnet being in the early stage of pregnancy when we last saw her. And Ash is still 10 years old.
Or Saturn, Mars, and Jupiter all escaped from prison and reestablished their Team Galactic HQ with little to no issue in the Arceus Special. And Ash is still 10 years old.
Or the ultimate example. Ash is in the Master 8 and hailed as the Alola Champion after 25 years of Pokémon journeys... and he's still 10 years old.
Time works when the writers want it to work in a certain way. And the same can be said about the vines.
I didn't know that the writers had to do things "your" way. Since when do we have any pull in what they do?
This back and forth is pointless at this point, i can only hope that if Greninja does not come back, you don't get upset.
Cause that is a possibility, not a certainty.