You can select the vertices and then hit S+Z+0+Enter. Then they'll have the same Z-Position.
S is the shortcut key for "Scale". Z is the the axis to scale on and 0 is the amount to scale by.
Or, if you wanted to align the vertices to a specific vertex, you can:
- Select the vertex you want to align to
- Hit Shift+S and select Cursor to Selected.
- Change the pivot point to 3D Cursor
- Select the other vertices and hit S+Z+0+Enter.
Or alternatively (without the need to change the pivot point to 3D Cursor), you can
- Select "Snap Element" on the 3D Header and select "Vertex"
- Then select all the vertices except the one you want to align to
- Hit G to grab
- Then hit Z to constraint the translation to the Z axis
- Hold Ctrl and hover the cursor over the vertex you want to align to until there's a circle over the vertex (don't hold Ctrl if the "Snap during transform" option is on)
- Then click (or hit Enter)