Is there a name for a direct liquid to gas transition which is not vapour in an intermediate stage? It seems the name 'vapourisation' is not suitable in this circumstance. Yet boiling is usually regarded as a type of vapourisation (along with evaporation).
By definition, a vapor is a type of gas, namely, a gas which is below its critical temperature (source: https://en.wikipedia.org/wiki/Vapor). A gas which is above its critical temperature can no longer be called a vapor.
A "pure" liquid state (i.e. one in which there is an insignificant* amount of vapor present) only exists below the boiling point of the substance. Crucially, the boiling point is always lower than than the critical temperature, except at the substance's critical pressure. If the temperature of the substance is between the boiling point and the critical temperature, any gaseous matter present is by definition a vapor. In order to go directly from the liquid phase to a non-vapor gas phase, you have to have a way to make the temperature change discontinuously, jumping instantaneously from below the boiling point to above the critical point. Since this is impossible, no such transition exists.
If the substance happens to be at its critical pressure, then the boiling point and the critical temperature are the same. But at the critical point, the properties of a liquid and the properties of a gas are identical, so the substance isn't really undergoing a phase change in the same sense.
*There is always some amount of vapor that exists above a liquid (how much vapor exists under given conditions is determined by the vapor pressure), and if you included that vapor, then your question would immediately be rendered moot, because even a liquid is always a liquid-vapor mixture. That said, for the purposes of the rest of the discussion we'll only count the vapor that exists in excess of the vapor pressure under the starting conditions of the liquid, because there are some valuable points to be made if we don't prematurely stop the argument.