I thought VANISH was not supposed to work so well in combat anymore, and I even heard that it is not supposed to work at all if they get entwined. I have had it happen 4 consecutive times now.

No bug?

Re: Entwine + Vanish, I've heard that too, but not from anyone who actually knows better.

As for the rest? RNG. It's not guaranteed to work in combat but it certainly can.