I've done it both ways but I suspect it's more efficient, though certainly more time-consuming, to neb first and then do the vest. The purpose of the neb is to get the mucus soft and flowing so you can cough it up. The vest facilitates the movement of the mucus out of the airways. If I have the time I'll do them separately, but if crunched I'll do them together. I don't see a huge difference in the outcome.