Do I need to be online when I use the app?
Only if you need to download a species list or one of the alternative layer systems. You can't look at maps if you are offline unless you have a cached map.
Will there be a version for iOS?
Probably not, sorry.
How to deal with permanent plots?
Vegapp does not distinguish between plots and plot obervations but you can assign a "Permanent Plot ID" to your observation
Can you provide me with a species list for my area?
You can ask via greens [at] vegapp.de but we cannot promise anything. The chances are higher if you have a synonymized checklist along with a valid citation for the list and a license that allows that usage.