Actually done by scaling, not transform. Procedure is:

- set pivot to cursor (main keyboard period)
- move 3d cursor to desired coordinate (e.g. another object that has the X coord you want for this vertex)
- select vertex to be moved
- hit 's' for scale
- hit 'x' to constrain to x axis only
- type 0
- hit enter

the vertex will jump along the x axis, effectively copying the exact x coordinate (only) of the target object.