Does a character using a Devil's Ride, the motorcycle-like vehicle from BG:DiA, count as "mounted" for the purposes of using a lance or the Mounted Combatant feat? If so, what about a larger vehicle; would a character be able to hang off the side of a Scavenger with his lance, or poke it from the gunwale of a galleon? Must a character be mounted astride a creature, RAW? A creature a size larger than its rider and with the proper anatomy "can" serve as a mount, but are mounts exclusively creatures? How is "mounted" defined in 5e terminology, as it is not a condition?
No you cannot "mount a vehicle"
On page 198 of the PHB it states:
A willing creature that is at least one size larger than you and has an appropriate anatomy can serve as a mount.
On page 216 of BG: DiA it states:
Infernal war machines are vehicles built in the Nine Hells and fueled by the souls of the damned.
PHB page 181-182 have a section titled "mounts and vehicles". The two are kept as different subjects. The Mounted Combatant Feat and the lance both specify the term "mounted". Therefore, An infernal war machine cannot be used as a mount.