OK, with VOF we track the free surface so a mixed boundary shouldn't add anything at all. For a mixed system that's going to separate (as above) I'd suggest reading up on the Eulerian or Mixture model.
Also, do not use all fixed flow boundaries. At least one boundary must be "free" to allow the pressure to stabilise. In a simple pipe, setting velocity in & and velocity out will result in vacuum or infinite pressure as the two flows mismatch by +/- 0.001% when running steady state: same for transient but it'll take longer.
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