I have been trying to find the event that handles all blocks updating, but can't find the correct one.

Please could someone point me in the right direction

 

PS. (When I say blocks updating I mean things like vines/crops growing for example)

I don't think there is an event for that. Block updates are handled inside the relevant block classes themselves.

I'm trying to recreate a plugin which stops things like vines growing, this was made for a custom map which had a lot of blocks that grew/changed in some way over time. This plugin helped reduce a LOT of lag in some areas. Any ideas?

Yeah.  There are ways that you can undo vines growing but that would cause additional lag, not prevent it.  You can also replace vanilla versions with custom versions of all growing blocks but that will cause lag during chunk generation so you'd be changing one type of lag for another.

 

In the end, Minecraft has a lot of potential for lag.  While people sometimes dismiss Minecraft as a "simple" world, the reality is that the number of blocks being processed grows is actually very large and the graphics which look simple still need full rendering.