Authentication is an important security aspect in integrated wireless networks. Different protocols are currently used for first and second authentication in integrated wireless networks. Authentication Certificates (AC) are used by Mobile Node (MN) for seamless roaming between integrated wireless networks. AC’s are helpful for seamless & secure second authentication in integrated wireless networks. This research paper is intended to evaluate the performance of AC based seamless vertical handoff. The Test-bed for this evaluation is comprises of NS-2 based GPRS-WLAN environment. We performed several experiments and the results proved that the AC based vertical handoff is seamless, it reduces network load and it ensures less time consumption for authentication at the time of vertical handoff. The results validates that AC based vertical handoff is seamless as compare to some of the other existing authentication protocols.