V-Ray Basic Material Properties setup in Modo

It's very simple, the name of the effect is the name of the channel prefixed by a material specific string.

"Reflection color" -> "vmtl Reflection Color"
"Reflection glossiness" -> "vmtl Reflection glossiness"
"Diffuse roughness" -> "vmtl Diffuse Roughness"

The only exception is the diffuse color in the V-Ray material, it will recognize both "vmtl Diffuse Color" and MODO's "Diffuse Color" effect.
We did it like this, because textures with the "Diffuse Color" effect are shown by MODO in its OpenGL viewports.

All effects for a particular V-Ray material are under their own sub-menu when you right click on the texture's effect.

Glossyness = Roughness

This is not exactly correct. Yes, reflection glossiness in the V-Ray material is equivalent to roughness in the MODO material, but it's in reverse.
So if you have a roughness texture from somewhere, you need to invert it, in order to use it as "vmtl reflection glossiness".

Is the reflection glossiness that's confusing me - is this for reflection, or glossiness, or both?

It's called "reflection glossiness", because in V-Ray you can also separately specify "highlight glossiness", which is the glossiness value when reflecting light sources.
You need to uncheck "lock hilight glossiness" to make this possible.
It's not physically correct, but I guess it gives you more control.

Greetings,
Vladimir Nedev

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Here is a practical example from my understanding
Let's say you bought wood textures somewhere and it comes with maps named, Wood_Diffuse/Wood_Reflect/Wood_Gloss/wood_Bump
You should use them like so,
Wood_Diffuse>>Vmtl Diffuse color
Wood_Reflect>>Vmtl reflection color
Wood_Gloss>>Vmtl reflection glossiness
Wood_Bump>>Bump
Like Vladimir said if the map is called roughness you will have to invert it and use it as Vmtl reflection Glossiness
Sometimes the maps you get might be named incorrectly and you can easily identify which map is which(see below). Usually you don't get anything for reflection color so you might get a Wood_Reflect which is meant to be used in the glossiness slot.
The glossiness map usually has more contrast unlike the reflection color map. In the screenshot below,on the right the this is the glossiness map and on the left, the reflection color map
If you got a specular map for wood or any material use it in highlight glossiness slot..it is not physically correct and the results are not good compared to other techniques. So don't do that
Use Reflection color and glossiness maps to get the best results.