Vulnerability and resistance

Smart Brail,

I’ve spent time with other sages to understand how to apply this tricky case and I guess to have the answer for you.

All is described in PHB p.197 , in particular:
“Resistance and then vulnerability are applied after all other modifiers to damage.”
Example: If your Skeleton take 20 damage from Bludgeoning, first apply resistance (10dmg) and then vulnerability (10dmg x2).

The total is 20, so your skeleton take normal damage.

Hope this help.