This is a much requested feature,Don't confuse with `edge slide`. and even the first of TODOs listed here:
https://wiki(This is a different area that can be explored later).
[[ https://docs.blender.org/manual/en/latest/modeling/meshes/editing/vertices.html#vertex-slide | Vertex Slide ]] is one of the variables of the transform that allows to move vertex along one of its adjacent edges.
Currently the only snap option that works in this operation is increment (which modulates the factor).
This patch adds other snap options thus fulfilling one of the TODOs in the [[ https://en.blender.org/index.php/Dev:Source/Development/Todo/Tools#Transform_Snap | TODO list of Transform Snap ]].