Vertex Editing
1) Within UnrealEd, select a wireframe render mode, or use a window already in wireframe for vertex editing.

2) Select the brush to modify by left clicking on an edge (this will allow you to see the current pivot point).

3) Select the desired vertex with a left click (right clicking will select the vertex, and snap that vertex to the grid -- moving the entire brush relative to the vertex).

4) With the target vertex selected, hold the Alt key, the left-click-and-drag the vertex.

NOTES:
- Grid snapping behavior is still not consistent -- turning off the grid-snap, selecting the vertex, then turning on the grid-snap, then moving the vertex usually works when nothing else does.

- Vertex editing will snap to adjacent vertices -- this is intended. Unfortunately, there is no good way (currently) to separate the vertices once they're aligned. (Vertex editing affects all vertices within the SnapDistance radius simultaneously.)

- A "too few vertices" error can cause the editor to crash if too many vertices are edited into the same plane.... I wasn't able to track down the bug before Unreal shipped. Sorry.

- The current implementation of vertex editing is not production quality (hey! I got it working in less than four hours. :) Make sure you save your work before you use the feature.