When your opponent activates a card or effect that would inflict effect damage to you: Your opponent takes the effect damage from that card or effect, instead.
Haven't tried this, but I would guess the answer is no. The text requires that the effect damage be due to a card your opponent "activates", whereas vassal's effect is continuous.
One thing you’re forgetting - Yubel’s special ability activates after damage calculation, not during the Damage Step.
But you are correct - a Counter Trap Card like Barrel Behind the Door is a good counter. You could also counter with the special ability of another monster, like the Herald of Orange Light Tuner Monster.
Thank you for visiting Duel Links wiki by GameA! We update this Yu-Gi-Oh! Duel Links community day by day to provide quality guides and the latest news.