This helped me a lot to understand the multi stage carbs like a Polini or Mikuni.
http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html
Yes it is describing larger carbs than a vm18, but all of the steps and understanding of them applies.
I will tell you that if you have a vm18 then you will need an idle jet around a 17.5 but, the main jet will vary depending on your setup. The tuning guide will help you know how to decide what size jet you ultimately need based on the plug chop at different throttle positions.
Have fun, it isnt a quick process, but very necessary if you want the performance.
I cant remember if there is a good plug chop image so here is this one.
