I don't think you're being thick at all as this is somewhat vague in CDS.
The PDF of CDS rules page 15 3rd example states an M113 would "get 4 Action Dice, the relevant number for a full squad of that unit type". I'm assuming this means a US Regular infantry squad. Any dismounted troops get Action dice based on their number of men from the actions table (section 4.2.1 on page 13), while the M113 retain 4 dice. The dismounted troops and M113 are reduced 1 dice for each movement dice used and 1 for de-bussing on the turn the infantry actually de-bus (section 5.3.4 page 22).
Page 15 2nd example seems to indicate tanks get 3 action dice, but the example is for an ARVN M41. Following the logic used for the M113, the "relevant number for a full squad of that unit type" would be 3 action dice for an ARVN regular squad.
Section 5.3 on page 23 also states, "All ground vehicles may use up to three Action Dice for movement whatever the quality of the troops within them. Any excess Action Dice possessed by a better quality unit may be used for spotting, debussing or firing."
To me, this implies that ALL ground vehicles would obtain as many action dice per turn as a full strength squad of the quality level/nationality of the troops manning the vehicle. This is how our group has always played it though it isn't explicitly stated that way.
Attack and Observation helicopters operate with three Action Dice. This is explicitly stated in section 5.4.2 page 23 and example on page 16.
Hopefully the page/section references help you decide how to play it.