for large peices i never use the peentool... i just cheat listen to this lol:
1) make a new layer above the reference image
2) get the "lasso tool"
3) zoom in about 600-700% onto the image
4) carefully trace around a section of shadow or light area with the lasso tool
5) get the "sampling tool" & sample that area you just traced/lassoed around
6) get a large the brush tool (about 70px should do it) and fill in the lassoed area
7) press Ctrl+D the deselect the lasso

get the erazer (100% hardness) and carefully lookand eraze any fawlts or errors you might see.
.... do this layer by layer & area by area.. and you will achieve perfect LP pentooling fast... great thing is you dont even need to have a steady hand
u know why? because i say so
but good work anyway i think you could have done more with it doe.. i mean rather than just vector it ..u could have added a few designs of your own to it i think