According to the commons model of cognitive radio, the activity of secondary (unlicensed) nodes is required to guarantee quality-of-service (QoS) constraints on the transmission of primary (licensed) terminals. Towards this goal, vertical handover between different radio interfaces is currently being investigated as a promising solution to enhance flexibility in unlicensed channel access. In this paper, we propose an analysis of cognitive radio with vertical handover capability in a simple scenario with one secondary node and two primary nodes that employ different radio interfaces with packet-based transmission. The maximum stable throughput of the secondary node is evaluated under maximum-delay QoS constraints on the primary activity as a function of system geometry, QoS constraints and sensing errors. Numerical results show the relevant advantages of optimal vertical handover in terms of the throughput of secondary nodes.