Allowing commercial service providers (SPs) to promote their businesses, ad dissemination in vehicular ad hoc networks (VANETs) shows great application potential. In this paper, a VANET-based Ambient Ad-Dissemination scheme (VAAD) is proposed to support secure ad disseminations with pragmatic cost and effect control. VAAD provides an incentive-centered architecture for the involved parties to trade off their conflicting requirements regarding ad dissemination. Given realistic advertising effect and cost requirements of an SP, VAAD adopts a distance-based gradient ad dissemination algorithm to maximize the achievable ad effect by emulating the ad-posting patterns in the physical world. To facilitate vehicular nodes' participation in VAAD, efficient, secure, and privacy-preserving incentive cash-in is ensured to support financial transactions in VAAD. Thus, with proper cost and effect control, VAAD is a novel and comprehensive solution to secure ad dissemination in VANETs.