The OP was not asking about what you think is a vehicle but how the rule book defines it in order to understand what certain instructions may mean. In this regard a vehicle is a single coach, wagon or locomotive, and this includes multiple unit trains irrespective of how their wheels and axles are laid out. Therefore a Cl321 is formed of four vehicles, a Cl156 of two vehicles and a 2+8 HST of ten vehicles.
The reason for this is to understand the rules when working out things like how to move a train which has some brakes isolated. Brakes are isolated on individual vehicles rather than across the entire train, so you need to know the proportion of braked to unbraked vehicles in the train as this dictates whether or not specific speed restrictions apply.
I’m sure someone who signs Stadlers will correct me on this, but my understanding is that articulated units such as these are still counted in vehicles. Precisely how these work within the rules will no doubt depend largely on how things like brake isolations are carried out and how the ratio of braked and unbraked is calculated. Given that these use Jacobs bogies shares between the vehicles it may be that brakes are isolated on individual bogies rather than individual vehicles and that this is the basis on which the calculation is based. If so, I’m sure you will find that this is stated in the traction instructions specific to those classes. In instances such as this, the rules set the principles which are then interpreted and applied to specific situations such as this.