This should be in the Ivysaur tactics thread. Oh well.
Yes, dthrow to vinewhip can be pretty handy against people who aren't suspecting it. Otherwise they can DI out of it fairly easily. I still bring it out every now and then regardless.
Keep in mind that the startup lag for vinewhip is much less when you are grounded. Despite this, using it in the air has the advantage of slightly increasing its range due to the small pop up ivy does whilst performing the attack.
Using vinewhip to punish approaches is difficult and risky due to the ending lag of the attack and the high probability of an airdodge/fastfall messing up what needs to be fairly precise aim.
Despite this, it works decently to increase ivy's zones of control. Once you establish vinewhip as a formidable defensive/offensive tool by whipping the opponenet as they come back on stage (or as they approach, or after a dthrow, etc), you have a nice little area diagonally up and in front of you that they wont want to be getting into. This lets you pressure them into approaching straight at you, which isn't too bad at all for ivysaur, thanks to razor leaf, bair and bullet seed.
Of course that is heavily dependent on the opponent's character and the opponent themself. All in all it's a handy little move though.