hmm.... if the tank is set up for fish only, keeping water to the top of the tank, this vine will wind up rotting in the tank, fouling up the water eventually. Make sure it's meant for a fish only tank I guess from the LFS. If it is grape vine, I would advise against the use in the tank however, it certainly is a cool looking piece.
As far as sinking it though, I have a piece of driftwood that has been sinking for 3 months now, when I remove the bricks from on top of it, it still shoots to the water surface. Some woods just take more time I guess.