The third-level party is currently in a stone tower; the floor we are on has a ladder leading up to a trapdoor in the ceiling, and we have reason to believe that the floor above us may be heavily defended. We would like to scout the area immediately around the other side of the trapdoor without exposing ourselves.
One possible plan involves having the Genie-patron warlock enter his Vessel (a dagger) and opening the trapdoor just enough to slide the dagger through and then closing it - this might not be noticed, the movement might be noticed but the dagger not, or the dagger might be collected as an unbeknownst Trojan horse.
Regardless, this question begins with our warlock in his Vessel, trying to scout a room for us that none of us can see.
The interior of the vessel is an extradimensional space in the shape of a 20-foot-radius cylinder, 20 feet high, and resembles your vessel. The interior is appointed with cushions and low tables and is a comfortable temperature. While inside, you can hear the area around your vessel as if you were in its space.
While unable to see, the warlock can at least be making Perception checks to hear anything around him, and he has the message cantrip from his Pact of the Tome.
The question is then whether he can communicate the information he obtains to us via his message cantrip while he is within his vessel.
The message spell has a range limit of 120 feet and further can be blocked by:
Magical silence, 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood
It is not clear whether the extradimensional space of the vessel counts as any of these things, or how one measures distance from within the space to the outside - Does the entire space fit in a point within the dagger itself, so that we measure the distance from the dagger? Does the 40 foot wide distance within the dagger count if the warlock is on the 'far wall' from the target? Does the boundary between our plane and the interior of the vessel count as an arbitrarily large distance?
Other communication spells like sending or telepathy have infinite range on the same plane but explicitly cannot be used across planes or from a plane to an enclosed extradimensional space. Message has no such restriction, but also has no instruction for converting its explicit 120 foot range across the boundary of the extradimensional space.
A good answer will be consistent with the warlock being able to hear what is happening outside the vessel; for example, if it is an infinite distance to the inside of the vessel, how do the sounds from the outside reach it?