I think asking for a referral is an extremely delicate thing in sales as well as raising capital. While you have little to lose from asking for a referral after a rejection, you also have much to gain from pitching this very carefully.

You don't want to come off as implying the VC is wrong to turn you down, consider acknowledging the possibility that your idea isn't ready in the VC's mind before asking for a referral.

"Hey Brad, thanks for your polite note... I get that it isn't right for you to move forward with our team right now. Quite honestly, I value your advice. Is there someone you know who might be able to help us move forward a little more, if not by investing at least by helping us understand where we need to get to and how to proceed? Someone who is perhaps specializing in ventures like ours?

"While we would love to raise money from someone like you right now, if we aren't ready we would appreciate steering us to someone who is in a position to help us sharpen our focus.

"Thanks in advance,

"Reginald S. Braithwaite, Esq."