Well I think you're over-complicating it a bit but if that's what you want to do, more power to ya.

The NPK of the vermicompost isn't really all that vital, it's the microbial colonies in it that play the largest role in it's beneficial properties IMHO.

Beyond that just look into the organic waste you're adding. Coffee grounds are high in N, so worms fed on coffee grounds will poop out more N-rich castings. Bananas are high in Potassium if I'm remembering right, so give them banana peels to boost that.

Personally, I'd say get a mild fertilizer if you want to have a particular NPK ratio to feed your plants and think of vermicompost as more of a soil supplement than a nutritional additive.

i dont think you are overcomplicating anything. as soon as i discovered vermicomposting i began devising methods of manipulating the compost to create tailormade castings as nutrient base for various plants. i see this is an older post, anything to report on the subject?

after ten years of i'm still dreaming up ways to 'manage' worms. most people involved in what may be described as counter-culture will never stop innovating or asking questions or making mistakes or discoveries. this idea that worms might change the world has created a world of it's own in my life and i'm always looking for ways and people to encourage this.

more to the topic - i use sheets of newspaper to moderate the moisture content in my winter bins. the 'castings' that collect look a lot like castings but a lot like ground up newspaper. so as, most of us would agree - input in = input out. but, as has already been suggested, the biggest benefit of worm castings is the herd of beneficial microorganisms - which can in turn be manipulated by actively accelerated casting tea.

i'm working on a couple of projects where this question has come up. so it's an interest of mine - so much so i might even sacrifice a pickle for the cause

cheers!

e.m. on the brain