I would focus on clear and open communication, educating them on Agile values and principles, to align our expectations. By integrating them into the team’s sprint planning, retrospectives, and review processes, they can better understand the iterative approach and collaborate effectively. Sharing specific use cases or examples from past projects helps them see the benefits while setting up small, manageable tasks can foster a smoother transition. This approach would help to ensure alignment with Agile goals and enhance collaboration between the development team and vendors.

Bring vendors on the same page as your core project team members, so they understand each other better and target the same delivery goal as one team. Create a continuous learning environment for vendors and set expectations on their participation and deliveries. Mostly, Agile learning comes from practice. Provide necessary Agile training and have vendors join daily standup, planning, review, and retrospective events. Working together and participating in the same events will help them speed up Agile learning. Regardless of theoretical training and education commitment is critical. Ensure the vendors understand the importance of the Agile way of working and adopt Agile methodology and its framework.

If non-agile vendors are best choice, we can consider three parameters. - Mindset : Easier to collaborate if vendor are treated as "Partners" and has shared purpose, goals and vision. Long-term, and transparent partnership will help foster environment of trust and open communication - Engagement : Basic knowledge on Agile and involving key representatives in ceremonies like PI planning, Inspect and Adapt iteration can bring a lot of difference - Result Driven : Setting expectation initially and measure them regularly basis will provide an opportunity to access and take right corrective action before it is too late !

Dealing with non-Agile vendors is like convincing an old-school chef to try fusion cuisine—it’s tricky but doable. Start by explaining why Agile works. Vendors often think it’s just a buzzword, so share real examples of how it reduces last-minute chaos and scope creep. Keep it conversational, not preachy. Next, focus on collaboration. Vendors may fear Agile equals endless meetings, but you can assure them it’s about streamlined communication and faster decisions. Sometimes, simply rebranding terms like "sprint reviews" as "progress checks" can ease their concerns. Expect resistance—it’s natural. Use humor to break the ice, share your own Agile struggles, and show them it’s about enhancing, not replacing, their work.

Best way to convince someone is to show the proof. If the proof is shown enough that it can benifited then they are almost onboarded . And for the starter do a pilot agile sprint to show then first hand experience of how agile can bring improvement in their way of working