Each triangle is its own entity, so a plain is basically 2 seperate triangles. So to move one of the corners you have to move 2 vertices. In your example, vertex 1 is in same place as vertex 3.
sync on : sync rate 60
make object plain 1,10,10
move object up 1,1
xrotate object 1,90
fix object pivot 1
set object wireframe 1,1
set object light 1,0
do
lock vertexdata for limb 1,0,1
if upkey()=1
movev(1,-0.1)
movev(3,-0.1)
endif
if downkey()=1
movev(1,0.1)
movev(3,0.1)
endif
unlock vertexdata
sync
loop
function movev(v,s#)
set vertexdata position v,get vertexdata position x(v),get vertexdata position y(v),get vertexdata position z(v)+s#
endfunction
Boo!