This study aims to support decision-making in vendor selection for water supply problem which is often complex and uncertain. Vendor selection involves multiple criteria, including technical capability, quality of proposals, financial strength, and risk management. Evaluating vendors in water supply problem is therefore a challenging task. To address this challenge, this research proposes a systematic framework for vendor assessment based on the judgements of technical experts, environmental scientists, local committees, project managers, and government authorities. Thereafter, we introduce a new integrated approach that synergizes power Bonferroni aggregation operators and Schweizer–Sklar operation with p,q-quasirung orthopair fuzzy information and grey relational analysis, providing a robust framework to tackle real-life challenges. Moreover, the proposed model integrates the MEREC (MEthod based on the Removal Effects of Criteria) and DEMATEL (DEcision-MAking Trial and Evaluation Laboratory) methods to derive more precise and reliable weights for both attributes and experts. This robust framework enhances multi-attribute decision-making by enabling decision-making to make well-informed choices and providing structured, accurate solutions that minimize uncertainty and enhance decision quality. What is more, using a numerical example based on the process of considering problem, we construct a novel decision-making method. Finally, we include sensitivity analysis and a comparison between the existing approaches and the proposed method in this study.