I think that this is not like the "does what you study in undergrad have to be what you do in grad school?" question. Think about what this would look like. A program wants to hire someone to work in one area; let's say, random paper in front of me, substance abuse. You apply, professing interest in that area. But, your thesis and dissertation are on vocational psychology with no hint of substance abuse. That's a hard sell.
But your potential research areas don't seem too dissimilar. e.g. if your goal is forensics, you might be able to study severe psychopathology in inmates at the schizophrenia research place.