Currently vassals will buy if they historically owned a province. However, this should change to Historically having a core. A core is at the moment an investment into a province, representing some sort of permanence. This seems like something they'd buy, rather than just a province they may have conquered once.
This also makes sense gameplay and roleplay wise. The current situation means that 1444 represents an absolute limit, with all history before that irrelevant. Lands that are cored but uncontrolled at the start of the game can not be sold back. This applies to living countries and countries existing only in latent cores. If say you have a vassal Burgundy or Bulgaria, you should be able to sell them any provinces which they once had a core on, but on which the core has been lost by time/culture conversion.
This also makes sense gameplay and roleplay wise. The current situation means that 1444 represents an absolute limit, with all history before that irrelevant. Lands that are cored but uncontrolled at the start of the game can not be sold back. This applies to living countries and countries existing only in latent cores. If say you have a vassal Burgundy or Bulgaria, you should be able to sell them any provinces which they once had a core on, but on which the core has been lost by time/culture conversion.