As an adjective, 'vast' describes something extremely big or large in volume, size or extent. You might have a 'vast knowledge' of a topic, for example; or have 'vast experience' in a particular area. As a noun, 'vast' refers to a space that seems endless. 'Vast' can be used in both social and professional contexts and is similar in meaning to 'endless', 'considerable', 'extensive' or 'expansive'.

1. The vast majority of people in this country want to see more taxpayer money spent on improving the healthcare system.
2. The company have spent a vast amount of money on building their new offices in the heart of London. 
3. Such is the vastness of Africa, it's hard for us to comprehend its true size. It's nearly three times bigger than Europe!
4. I presume I got the job because I was vastly more experienced than the other candidates.

1. The company have spent a vast amount of money on building their new offices in the heart of London. 
2. Such is the vastness of Africa, it's hard for us to comprehend its true size. It's nearly three times bigger than Europe!
3. We require a candidate with a vast knowledge of programming languages.
4. The company collapsed because Management vastly underestimated the consequences of their bad marketing strategy.