In this paper we investigate the potentiality and the benefits of a soft Vertical Handover (VHO) mechanism, compared with the traditional hard approach. More specifically, we present an analytical scheme for seamless service continuity in a heterogeneous network environment, modeled by means of a multi-dimension Markov chain. The call blocking probabilities, as well as the soft and hard vertical handover probabilities, are computed for specific networks (i.e., UMTS and WLAN). We propose a soft/hard VHO technique working either as (i) a Mobile Controlled Handover, on the basis of a reward and cost model, which consider the data rate and the bandwidth allocation, or (ii) as a simple Network Controlled Handover scheme, by assuming a probabilistic approach as the handover decision metric. Simulation results validate the benefits of the proposed handover algorithm when operating in soft mode, which outperforms the traditional hard approach in terms of network performance and limitation of unwanted and unnecessary handovers. Finally, the effectiveness of the proposed approach is proven with respect to other single and multi-parameter VHO techniques, by extensive simulations.