It sounds like you might be looking for the Swizzle node. According to Valve's documentation:
Uses arbitrary swizzling on a vector. You can also swizzle with constants and change the vector dimensions as you see fit. Use the properties dialog to define the swizzle operation, supports token channels.
So to take an input vector (x, y, z) and select just the y component, you'd place a Swizzle node by right-clicking and selecting
New node -> Math (Vectors) -> Swizzle, then double-click it to access its properties and enter the string "y" to output just the y component as its own pin.
You can add additional Swizzle nodes to split out the x and z components too, to have three separate paths in your graph for the three components of the vector.