"Paul, when does an early stage VC start to feel comfortable with an investment?"
Early stage VCs regularly evaluate the companies in their portfolio. They do this to decide if they should continue pouring time and money into a particular company, or if they should shut it down and swallow a loss.
This type of evaluation often starts with a rough measurement of the risk involved with the company. For a VC, a huge amount of risk is eliminated when you've achieved what I call "cruising altitude". Cruising altitude is the point at which a VC is confident that at least he won't lose all of his money. In other words, at that stage, the VC believes that your company could be sold to someone (often just for the technology).
When you've hit that point, the scariest part (takeoff) is behind you. So don't put the VCs money at serious risk and you're halfway there. But the flight definitely isn't over, and your focus needs to shift to executing a safe landing (read: exit).