Software‐defined networks (SDNs) have proven to be highly flexible for vehicular ad‐hoc network (VANET) routing. This is due to packet‐level and bandwidth‐level controls available with SDNs. But SDNs have inherent security issues which include forwarding device attacks, control plane threats, communication channel vulnerability, etc. To counter these attacks, this paper initially proposes a proof‐of‐work‐based blockchain framework that can reduce attack probability by 99%. But, the overall network quality of service (QoS) reduces in terms of end‐to‐end delay, energy consumption, and throughput. To improve these parameters and maintain a high level of security, this work proposes a machine‐learning algorithm that optimizes blockchain parameters in a QoS‐aware manner. These parameters include selected chain length, encryption block length, etc. The machine learning algorithm is inspired by Q‐Learning and aims at reducing the performance effect of blockchain operations on the overall QoS of VANETs. Thus, the underlying network uses sidechain‐based blockchain implementation, which makes it faster, more secure, and less complex when compared with single‐chain implementations. Upon observing the results, it is seen that the proposed architecture can reduce the end‐to‐end delay by 15%, energy consumption by 18%, and increase overall throughput by 38% when compared with existing blockchain‐based SDN VANET architectures.