Start by clearly communicating the project's critical timelines and non-negotiable requirements. Show understanding of their constraints and collaborate to identify mutually beneficial solutions, such as phased deliveries or resource sharing. Build trust by offering reasonable flexibility where possible, like adjusting less critical aspects. Regularly monitor progress, maintaining open communication to address issues promptly.

Firmness and flexibility as opposites on a spectrum are often confused with another dimension: appreciation or respect. But being firm doesn’t mean being rude, and being respectful does not necessarily imply becoming flexible. Besides that, balancing firmness and flexibility effectively can be done by shifting our focus to explore the underlying interests behind the positions — both yours and the vendor's. Why do we feel the need to stand firm on certain points? What would it take to yield, trade concessions or compromise? What pressures or challenges might the other side be facing? What else could help them? Such an understanding often reveals alternative pathways. Creative solutions suiting both parties' interests become more likely.