Not sure on the nature of the flying you do/intend to do but while VNAV is complicated I find it becomes much less so (as with anything) when you fully understand the principles. Appreciate that's what you are trying to do on here wth the question but in reality the FCOM's are your best friend. If it's for flight sim and you only require a basic knowledge fair enough but don't be surprised if you find yourself asking 'what's it doing now?'...regularly!