The fatigue damage of Sleep Vines is armor-negating, I'm pretty sure.
They're brutal, en masse; if you're an SC with, say, four squares of two-vine mandragoras attacking you, that's 4 sq * 3 m/s * 2 vines * 3 att/vine = 72 vine attacks per turn... and the usual wraith sword won't be able to restore any fatigue by draining them. They still might not hit that often, since they're not exactly great warriors, but perhaps often enough...
Defense will help somewhat, but against that swarm of attacks you'd want a damage shield to kill the mandragoras and keep the number of attacks down; in particular, an astral shield should paralyze the mandragoras before they actually check to hit you, IIRC. The vines are also long, making it tough to repel them (either length 4 or 5, IIRC; if length 4, then a Dancing Trident, Machaka Spear or similar weapon should be able to. I don't know whether a Banshee's Wail (length 6
) can repel.)