I would say that Eva is a classic that is trendy right now. It's gone up 275 spot in the past 15 years but it never got that low. I pronounce Eva with a long E and Evelyn with a short E so Evie wouldn't be a logical nn for Eva. My grandmother is Evie and it's short for Evelyn. If I heard Evie I would assumed that's what it was short for.
Erm, how are you saying Evie? Because most people I know say it like EE-vee. So Eva nn Evie makes perfect sense. Evie said like EV-ee is done, but it's not as common as EE-vee, from what I've heard.