My step-daughter who is nearing 18 and currently out of UK

I'd like help finding a site or material that would help me understand, simply, the situation with my step daughter.

My step daughter holds ILR as daughter of my wife who has ILR. She will be 18 in March 2016. I, my wife and my step daughter are currently living in Viet Nam as I can work here ( I am 65). My wife and I will return to the UK in Summer 2015 so that her ILR will remain valid.

Because my step daughter's education has been disrupted by the rigid VNese system, it would be better for her to return to the UK in Summer 2017, when her VNese education will be complete.

I need to know
a) if that will be a bar to her rejoining her mother,
b) if spending some time (and how much time) in the UK would "reset" her ILR so that she could return to VN to finish her education,
c) whether adopting her would make a difference.

Finally, if my wife and I stayed here in VN until 2017, (thus losing her and my step daughter's ILR) what will be the prospects of my wife being able to rejoin me in the UK ( I would then be 68) and of my by-then-adult step daughter being able to follow her mother.

ILR isn't "lost" as such - but it can expire if the holder is out of the UK for more than 2 years.

AIUI - your step daughter is no longer a dependent of your wife as she now holds her own ILR - thus if you extend your wife's ILR - that's independent of your step daughter.

If either remain outside of the UK for more than 2 years they will need to apply under the returning resident provisions:

https://www.gov.uk/returning-resident-visa

Thank you