Problem definition tree diagram, Goal-means diagram and decision tree diagrams can serve the purpose of CTQ drilldown. When VOC is vague and not specific, Quality Function Deployment (QFD) can be used to translate VOC into measurable goal statement.
For example, a customer can ask you to "improve your system". Improve your system is a vague language that can be translated as more productivity in terms of output, faster productivity in terms lead time, better quality delivery with reduced defect rate and cheaper productivity and cost of quality.
It is the three approaches or tools mentioned above that can help you define a specific goal to satisfy your customer.