I agree with the previous poster, I think spelling it Evie will stop the endless spelling it out for people all the time (I know as I have common name with an alternative spelling and it has always been a total annoyance). She will be always be unique even if there are a couple of other Evie's.
If you really want to be different, you might just need to choose another name.
Rx
Same - I have the less common of two spelling variants as my name and it is NEVER spelt correctly (I've given up the battle now and just let them spell it how they want) - and it's annoying. Plus I just hate mangling the spelling of something to be "unique" - if it's popular, it's popular, if that's an issue - pick something else, or hyphenate it or whatever... and yes, I'll go there, the unique-ified spellings DO have a certain um... aura... they carry to them and people will make assumptions - I just don't think it's fair for a child to have those assumptions placed on them as something to carry just so they weren't one of two with the same name spelt the same in a class ten years ago.
I'd read what you're planning as Eye-vee btw.