If my army represents unit from one legion it wouldn't be historically accurate to have each unit with a vexillum standard, correct? These were used for units detached from a legion such as individual cohorts operating independently. So, if I have 10 units of legionaries, each representing a cohort from the same legion, technically I should have zero vexillum standards in my army?
No, you'd have one for the legion. Each cohort has its own signifer. Each legion has its own vexilla. If your army has the 10 cohorts from a single legion then the Vexilla will be there too and probably its eagle. You wouldn't really have one for each cohort from ten different legions.