And if I snatch it through the bloodspine wall, it gets the attack too right?
I presume that "it" refers to the wall. If so, then yes.
You activate the Thornlasher, and attack with its Ensnaring Vine attack. You will roll the three dice (base) attack and roll the effect die. If you make your five+ (after any possible modifications) you may then pull the target through the Bloodspine Wall, doing its three dice, piercing +2, unavoidable attack with a good chance at a bleed. Then, as others have suggested, you can or must (depending on what it is) activate other attacks from conjurations or enchantments already in the zone. If the creature targeted has little or no armor, then it can be quite deadly.
You don't get any of those extras though if you don't successfully Snatch to begin with... you'll only get your three die base attack and the target will remain where it is. Note that the snatch will automatically not succeed under certain other circumstances... Eagleclaw Boots grants Unmovable, just as some creatures are (Iron Golem, for instance), and any thing restrained by a Tanglevine, etc. will be able to ignore being snatched.
