You would be vulnerable to the damage
The second half of the Overchannel feature states:
If you use this feature again before you finish a long rest, you take 2d12 necrotic damage for each level of the spell, [...] This damage ignores resistance and immunity.
It only ignores resistance and immunity, not vulnerabilities. Though multiple resistances and vulnerabilities to the same damage type will usually cancel each other out, this isn't always true. This answer supports that both would be applied if you had them.
The rules on damage resistance and vulnerabilities only mention cases where you have multiple sources of resistance, these do not stack and would be ignored in this case anyway. The best rule it gives us is:
Resistance and then vulnerability are applied after all other modifiers to damage.
So first you would apply your resistance, which is ignored, then your vulnerability. The result is you take double damage from this feature.