Vanilla is a product of Lussumo:
Documentation and Support.
"We want you for a secret deep sapce mission where your skills may be needed. You will be briefed on arrival. If that is acceptable to you, take this large briefcase filled with more money than you will make in ten years and we'll see you at the launchpad."
David is fully dedicated to the mission. As he understands it, the mission is to figure out what the intentions of the Engineers were ("asking god why"). As they explore the ruin, it becomes clear the Engineers intended to infect humanity with the black goo. David has access to both the black goo and a small sample of humanity. David runs the experiment. Shit starts xenomorphin' and nobody knows why, they don't know who is infected, but they have to figure it all out before everyone is dead.
There was nothing groundbreaking visually in this. The ships looked like generic science fiction ships. The alien stuff looked like extrapolations from the original art design of Alien.
For all we have been told they are the only ones going out there. The mission is on Weyland's private ship, funded with his money, all the people are his employees. The film has not given us the slightest reason why any of that needed to be a secret from the crew, or from anyone.It's implied pretty heavily by Vickers in two different scenes that the Board isn't terribly happy with Weyland blowing a trillion dollars of the company's money to investigate the crackpot theory of two young archaeologists in the hope that he'll be able to extend his life for an indefinite period of time. If that information were public it would be disastrous for the company's stock.