Photoshop EPS and PDF not saving vector mask when opened in Acrobat

Not sure if this is a Photoshop issue or Acrobat issue. I have a Photoshop file with a vector mask on a layer. I'm placing this image into an Illustrator doc and then saving the Illustrator as a PDF. No matter what format I save the Photoshop file in — PDF, EPS (with "save vector data" checked) or PSD — when I open the Illustrator PDF in Acrobat the vector mask is rasterized. If I open the Photoshop PDF directly in Acrobat (i.e. before placing in the Illustrator doc), the vector mask is also rasterized. Saving as a Photoshop PDF or EPS used to preserve vector data...