The best rule of thumb here is "maybe means no".
["that" being a wishy-washy response, no contact for weeks etc]
Here's a post I wrote about this ages ago: http://www.derekscruggs.com/slow-turndowns-bad-up-front-cont...
I've found that giving people permission to say "no" is powerful in many contexts, not just sales.
Understand that some (many?) VC's understand that saying "no" explicitly can hurt your prospects with other VCs (you will be asked to explain why that other VC said "No" which is impossible to do usually because you don't have the complete picture). So they let it drift quietly silent and understand when "you are really busy and might not get back to them for a while."
Another tough thing to balance is the flip of this - how to politely drop a VC you do not want to talk to further, who is interested in your company and knows you are fundraising...
The logic is simple - if the original VC is waiting for a new VC to get excited about it, it would not matter if the original VC is still in the mix or not (as the entrepreneur now has someone else interested). If no one else gets excited, spending time on the original VC is a waste of time, since she won't tip unless someone else does.