Look first at proven experience, not the number of logos on the website. Ask to see two or three projects that resemble your technology stack, and then ask specifically whether those engineers are still with the vue.js company. A solid partner will introduce you to the engineers. Answers that name nobody at this stage generally mean the delivery team is not the team you were shown.

The agreement deserves more scrutiny than the proposal. Three clauses do most of the work: assignment of intellectual property, the NDA, and termination and handover. All the work product has to transfer to you once invoices are settled, together with documentation, pipelines and deployment scripts. Watch for language that keeps reusable components in the vendor's hands, since it is usually the part you cannot replace later.

Ask how they estimate. A serious estimate is accompanied by a list of assumptions, a task-level breakdown and an explicit range. A fixed price is only reasonable when the requirements are stable and documented; in any other case the supplier prices the risk in and you fund the buffer regardless. Time and materials moves the risk back to the client, so it needs visible weekly reporting and a spending cap.

How the work is run matters as much as the number of developers. Establish what happens when the scope changes, who signs off on a feature and what the QA setup looks like. A well-run team will be able to walk you through a working build every one or two weeks. Clear, written acceptance criteria remain the only reliable protection against the it-was-never-in-scope conversation.

Last, consider the day you no longer need this vendor before it becomes urgent. Require that the repository lives in your organisation from the first commit, difference between monolith and microservices that documentation is updated as part of the work. A partner who is comfortable with this will agree quickly; a long negotiation over it reveals a great deal.