I have grinded the C172 cargo missions and now want to go with the Vision jet VIP missions. A few questions:
I cant buy a vision jet even if i got enough credits
I have all my certifications.
Do i need the specialization VIP charter and charter service private completed to buy this plane for my company?
What is the fastest way to grind the VIP charter and charter service private missions with B or above? (simrate, skipping etc)
If you click on the vision jet on the store it will tell you the ratings/prerequisites you need to purchase, if you haven’t done the golden vip mission yet you’ll need that too and you’ll need to start a charter company
I think wcc96387 has given you the answer you need regarding buying the jet.
The higher speed offered by the jet will make the missions go quicker so I never feel the need to skip.
I would just add do check the details of the airstrip you are flying to as some of them can be rather short!
I am quite picky with the VIP charter missions I am accepting for this reason 
and I bought a Beech 58 for fly these “STOL” missions