I can only see vertex colors in Vertex Paint mode - otherwise gray I'm using Blender 2.81 beta. I imported a large glTF model which has only vertex colors. (I can see them in other glTF apps like the web viewers.) After the import completes (it takes a while, 7M verts, 14.7M faces, 182 objects) it's all gray. No vertex colors visible -- neither in the viewport, nor EEVEE, nor Cycles.

However, if I select one sub-object and go into Vertex Paint mode, the vertex colors show up on the active object! (If I select more than one, only the active one gets its colors.) So I know they're in there -- they just don't show up. And I can vertex-paint them out. But they just never show up in the render.

Do I need to do something to get vertex colors to show up?
Figured out a workaround. These objects don't have any material in the glTF, just the vertex colors. So I created a material on one object, basic Principled BSDF, and as the color input I added Vertex Color node with the color set to COLOR_0. Then I linked that material to all the other objects, and it seems to work.

A bit painful, not sure why the importer can't just do that, but it does work for me.