Generally, the poison bite attack targets vital defense, but I consider them as having two different effects. At my table it takes an attack vs MELEE to hit with the bite attack. If the attack is successful, on the next round, the attack is rolled a second time against VITAL. On a successful hit, the opponent gains a 1st degree Poisoned Condition (or 2nd degree, if the attack Crits.).
Poison would be a trait added onto an attack, like a bite is Piercing and Poison (which can be done as an upgrade) and the victim is only moved on the status track when hit with a Critical, therefore Vital defence never comes into the equation.