You can easily convert metallic textures to IOR maps.
- Select the metallic file texture node.
- In the Color Balance section turn on the Alpha Is Luminance box.
- Create a ramp texture.
- Connect Out Alpha channel (metallic texture) to ramp V Coord channel.
- Set the ramp values.
(For exapmle: first value is 1.46, second is 13. 1.46 is a usual plastic material, 13 is silver.)
- Connect the ramp (Out Alpha) to IOR.
Use the Fresnel (everywhere in Vray).
Amount of reflection:
You certainly need to adjust the glossiness ramp values.
In theory the Reflection Color texture (aka Specular) take care of the Amount of reflection by itself.
Feel free to set as you see fit. It depends how the texture is generated.