In the new area of heterogeneous environments, roaming users, using multi-interface mobile terminals, need to access to services anytime and anywhere. For that, a seamless vertical handover has to be considered. In our paper, we propose a 3G/UMTS-WLAN loosely-coupled interworking architecture with mobile IP as a service continuity solution. We focus on a mobile-controlled vertical handover management scheme. Our solution chooses the best access network according to user preferences and it maintains the ongoing session. It has several advantages to be simple and to generate minor changes at the network side. Moreover, MIP-based handover execution procedure is prepared thanks to a handover decision taken at the mobile side. The scheme proposes an interesting answer to intersystem and inter-domain mobility management issues. Finally, a simulation model is developed to validate the feasibility of the proposed 3G/UMTS-WLAN architecture.