I try to make a material with smooth transparency distribution, but in my version softness and distribution don't work. How to do it right?

you are taking a 3dimentional vector into a one dimentional float, how could that work as expected?

How to do it right?

You should use float ramp attribute instead of vector(color) ramp as transparency is float !

Also you should use a float value between 0 to 1 as a input of ramp attribute not vector.

In your example you can use vectorTofloat vop to extract P.y and use it as input of ramp.
You may need to remap range of P.y between 0 to 1 by Fit Range Vop.

Another idea is using Relative to Bounding Box vop node ,this node give you a value between 0 to 1 according to xyz position of points.

Any normalised coordinate will do. Attached file uses the V of the UV coordinate and will act as the vertical axis.

Note when you use a vector to a float input, Houdini will simply use the first vector element as a float.