This is one of those monsters that sounds interesting at first, but turns out to be yet another example of a well-known trope. The assassin vine is no exception.
On the one hand, it's pretty good at locking people down. High strength, high constitution, large size, improved grab, the ability to create an Entangle effect all around you, and 20 ft. of reach. Sounds great? Well, here's the downside: you have a move rate of five feet. That's right, an elderly gnome with a leg injury will still easily outpace you.
Even if you find a way to move around the battlefield, there's still the problem of you being a freaking plant. Armor will have to be tailor-made (I'm imagining some kind of chainmail sleeve), most magic items can't be used by you, and even Gloves of Man won't help you out, as you lack tentacles or paws. You have a single natural attack that's honestly pretty poor, but getting anything else will require heavy resource investment.
On the racial HD: those could be worse. The poor chassis is somewhat compensated for by the immunities you get, but if you really care about immunities, why not go warforged or necropolitian?
In the end, I think LA +0 is best here. Being unable to use most magic items in a game where they are more-or-less mandatory (especially for martials) is bad enough, but the fact that you're as fast as a beached mermaid kills most AV characters. Having to rely on the DM putting monsters within 30 feet of you isn't a viable tactic for any character. Don't get me wrong: someone can make this work, but they don't deserve a LA on top of their obvious problems.