from what i know VCs are investors and not donors - they want something in return (always) ... nevertheless you may find few VCs who may want to donate on personal basis. OR you may want to tweak your business/charity model where investors get something, like (90:10 charity:business) model. Also big VCs might have some % of profit spent on charity just to give back to the society, for that you need to contact all big players. The best idea is to get couple of VCs/angels on your management team (or board) - they will raise more funds (just by making couple of phone calls) than you can ever get it by knocking doors for the whole year.

EDIT: the another idea is to put "Donate" button (asking users to donate by credit card) on almost all the web pages - more the people use your service the more the donation coming in. you may also raise some money by putting ads (text and display, related to your service) on the web pages, for example if you're providing online service related to health care then you can get advertisers from health industry (or if u want to keep it simple then use Google's and Yahoo's ad services).