An estimate that arrives instantly is a warning, hire dedicated react js developers not a service level. Any serious team responds with questions first: about users and volumes. A provider that commits to a figure without asking anything is simply guessing, and that guess becomes a change request later — and you will pay for it.
Be wary of any distance between the team in the pitch and the people who will code. Ask for the names and CVs of the actual team in the agreement, with a clause about substitutions. A provider that will only describe roles and refuses to name people is keeping the right to assign anyone it likes.
Ask for the source repository from the start. A provider that hands over code only at milestones is inviting you to trust a black box. Regular commits and pull requests tell you the actual pace far better than a weekly report. This extends to the CI pipeline: if there is no pipeline, quality claims remain nothing more than words.
Ambiguous contract language around code ownership is not a formality. The document needs to state explicitly that all deliverables belong to your business as they are paid for. Look too at which country’s law applies and the milestone terms: a large upfront payment with no milestone tied to it eliminates the only leverage you have.
Lastly, look at communication. Ask how much working-time overlap you will share with your timezone, which is better vue or react person answers questions and within what time. A few hours of overlap is normally sufficient; none at all converts each small question into a twenty-four hour round trip. Unclear written communication in the sales phase does not improve under delivery pressure.