First of all, sorry for the late answer.
You have a sample of custom shaders given with the plugin (Sample_CustomShader.unity). You will find more information about custom shaders here:
You will indeed have to code directly in HLSL, but it should not be that difficult. You will find a sample for lit particles in the Sample_CustomShader scene, the light computation is made in the fragment shader, but you should be able to easily move it to the vertex shader.
Hope this helps,
do not hesitate if you have any additional questions!