How do non-sublimable solids exist in equilibrium with vapour?
Thermodynamically, even cotton exists in equilibrium with its vapour, even though this is completely silly in any practical sense. The reason is that thermodynamics does not involve time; it may take $10^9$ years, but so what! The equilibrium may only consists of one molecule / km$^3$ but $technically$ its still equilibrium! This is because you can't arbitrarily pick and choose the cases you want thermodynamics to apply to, but you can apply reasonable physical insight based on observation rather than applying theory blindly.