How to get&set position of vertex in MEL?

use the xform command, for example:
create a polyCube and enter
xform -a -ws -t 0 0 0 pCube1.vtx[0];
if you want to know and get the position of a vertex:
float $vtxPos[3] = `xform -q -ws -t pCube1.vtx[0]`;
print $vtxPos;