vanity separated from shower by doorway and toilet
Water supply pipes on an exterior wall have a greater chance of freezing. 

We do this all the time where I live, but we make sure the building insulation is on the exterior side of the water supply lines. Make sure the bottom and top of the insulation batts are turned so as to keep cold air from seeping into the cavity.

You’re not moving the lav that far, so venting, pressure drop, etc. is not an issue.