Hi,
Thanks for your question. When you copy a card, you copy the card as printed (and as modified by other copy effects). You do not copy other effects, like the effect that made the original vehicle into a creature, for the copies. The copies made by myriad will be tapped noncreature vehicles that are not attacking (since they are not creatures). They're free to crew them, but it's too late for them to attack during that combat phase.
706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The “copiable values” are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and “as . . . is turned face up” abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.
Thanks,
Nathan Long
Magic Rules NetRep