Why VCs don't sign Non-disclosure agreements (NDAs):

1. Active funds receive hundreds of business plans each year.  They simply have no time to sign confidentiality agreements.

2. It's not their business to steal your ideas.  VCs are in the business of making money by turning their investment in your company into an exit that earns them and you an obscene return.  They really don't care what the technology or idea is as long as it is one that makes money.

3. NDAs are an obstacle to syndication. Many VCs will want to syndicate with other funds.  This spreads their risk and also helps bring in a broader group of experts to the company.  An NDA would simply make it impossible to talk about the investment with other potential syndicate partners.

Trust me, most VCs assume the technology works, what they care about whether someone will buy it.