I found that it was good to skim the vignette to get a sense of what was going on, then read the questions to find out what they want to know. Then you go back to the part of the vignette to get the actual numbers you need for calculations. Often the qualitative stuff you can pick up on a skim, as long as you flag any key issues along the way.
It’s rare that a question requires the entire vignette to answer (though it happens). Often the information needed for a question sits in only one or two paragraphs of a page-long vignette, so you know where to go to get the details.
In theory, it’s best to read the questions first, then go to the vignette, but I found in practice, it was too hard to understand the context of the question without first skimming the vignette.