It's theoretically possible to do it in Vue but it's a lot of work and it's very delicate. Adusting the iris/pupil sizes is not just a matter of material scale adjustments. The iris is a separate object and so is the pupil and so you'd have to adjust the object scale of the iris and of the pupil and you'd have to make sure they still match nicely with the cornea and the eye white.

(note: all these objects will be imported into Vue as one single eye object; to separate these into separate objects, you'd have to double click on the eye object and click on "split" - then you'll have 4 different objects, grouped under "eye").

Anyway, it's too delicate to do this manually in Vue. It'd take a long time to do it right. In Poser it's extremely easy to do it, just a slider.

Maybe you don't know that if you import a Poser object into Vue and later on change it in Poser and save it with the same name, Vue will ask you if you want to reimport the Poser object. It will do it into the exact same space as you placed it in Vue, so the workflow is extremely easy. Just change the iris in Poser, save the file, allow Vue to reimport it and there you are.