Vertex Blending: Multiply a texture with a vertex colour?

It should work. Are you 100% sure the alpha channel has information in both the vertices and the texture?

Yep the texture alpha and the vertex alpha channel definitely have info there, but the problem is with using the texture alpha channel with the vertex alpha channel - Multiplying by R,G or B works, just not the texture's alpha channel for some reason. Also, when you hook the vertex alpha Texture Alpha up to the emission on it's own, it fails to display it - possibly the root of the issue?

Note that this is on an unlit shader, if that makes any difference

Edit: tried this on Lit, Phong etc. and on different outputs. The texture alpha channel is displayed in the shader editor, but not in the engine.

Could you paste the shader into http://hastebin.com/ ?

(With the same setup that you've got in the screenshot)

http://hastebin.com/bujarihenu.sm

Sorry this is the source (other had vertex RGB linked, not alpha):

http://hastebin.com/pevifunose.sm

I'll check it out as soon as SF is compilable again! I'm in the middle of making changes :)

No problem. 

Also noticed another odd bug: One Minus doesn't work when plugging a single channel in from a texture

This is odd, I'm not having any of the issues you are mentioning, everything seems to work fine on my end. What is the simplest case where it goes wrong with the one-minus case?

Also, are you sure you have assigned the textures to the actual material that you use in-game?

I've put together a package with the problem shaders which you can take a look at here: http://www.littlelostpoly.co.uk/PVT/ShaderTests.unitypackage

On closer inspection it all seems to stem from disabling "Receive ambient lighting" on unlit materials

This is what I see on my end:

Ah, I see :) Thanks a bunch for the package!

This is all due to the master node not filtering the input. 

You're sending in a Vector 1 / Value into the Emission, instead of a Vector 3. It works when you have ambient light on, becuase the ambient term is added together with the emissive, which returns a Vector3. If you want a temporary fix, you can do this:

I'll try and get this sorted for the next version :)

After Multiplying all emissive with Vector3(1,1,1): 

ok thanks, will use that as a temp fix for now! Glad we got to the bottom of it!