Since Open Radio Access Network (OpenRAN) separates the traditional monolithic RAN component and legislates integration through standard open interfaces, it is regarded as a novel way to deploy 5G. This paradigm gives mobile operators the freedom to procure multi-vendor solutions, reducing vendor lock-in to the advantage of innovation. However, seamless interoperability of equipment in a multi-vendor environment remains a key challenge. The paper reflects the design considerations, field trial experiments, and lessons learned during the evaluation of multi-vendor Open RAN deployments. In a commercially neutral laboratory, E2E systems combining multiple DUs, CUs, and RUs, as well as RAN intelligent controllers from a variety of vendors, were set up. Interoperability was tested primarily on the critical FH and E2 interfaces because they were the hardest to integrate. Those trials in the field proved fruitful, as more than two-thirds of the tested setups achieved successful E2E integration after massive upgrades in software, interface parameter alignments, and backward compatibility compensations. This report highlights practical observations about the maturity of the present Open RAN products, bottlenecks in achieving plug-and-play interoperability, and the collaborative efforts needed among vendors. These findings may heighten guidance for speeding up Open RAN evolution and building 5G networks that are robust, scalable, and interoperable.