Specifically in regard to the effects of a hit with an adamantine weapon, would a hit with one turn into a critical hit? I know it's a vehicle but it's powered by souls and has a stat block like a creature would have. I'm going to be DMing Descent into Avernus soon and I want to make sure I look into anything that could come up during play.
Yes, Infernal War Machines are objects. Allowing for adamantine weapons to score automatic critical hits.
In the Player's Handbook, in chapter 9, under the section 'Making an Attack' (page 193), there are 3 listed options for targets: a creature, an object, or a location.
In the Dungeon Master's Guide, in chapter 8, under the section 'Objects', the subsection 'Damage Thresholds' (page 247) says:
Big objects such as castle walls often have extra resilience represented by a damage threshold.
Under the subsection 'Hit Points' (page 247), there is a table of suggested hit points by object size, within which is a cart, listed as a land vehicle, meaning that vehicles are objects.
Also in the Dungeon Master's Guide, in chapter 5, under the section 'Unusual Environments' (page 119), there is a table of vehicles, most of which featuring a damage threshold, reaffirming that ships and other vehicles, especially those with such a threshold, do count as objects.
Each of the vehicles present in Descent into Avernus is also listed as a vehicle, and also lists a damage threshold alongside their hit points, which means they would also fit under the category of 'object' for attack and damage purposes.
They are vehicles, but whether or not it is an object may be up to a DM
The stat blocks for them list them as vehicles so that we know. But whether or not a vehicle is an object has been debatable on this site for awhile.
The DMG describes objects as :
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
A vehicle is clearly not a discrete object. It is made up of discrete objects, but a vehicle is much more complicated than that. The descriptions later on about damage thresholds, etc. may apply to vehicles, but that doesn't make them equivalent to things like a castle wall - which is clearly a discrete option.
Some DMs may differ
And that's okay! At times, it may simply be easier to say they're an object. It may result in unintended mechanics, but if it works for your table, then it works for your table.
Based on the prior rule cited above, I do not believe they are discrete objects and would not make complex things like a vehicle active targets as a whole for mechanics that require objects and would not say that an Infernal War Machine (a vehicle) is an object.