Look first at relevant experience, not the length of the client list. Ask for three or four engagements that sit close to your domain and your stack, and hire protobuf developers then ask who actually wrote that code. An honest provider is happy to connect you with the people who would work on your project. Vague answers at this stage usually mean the demo work came from somewhere else.
The contract needs more attention than the sales deck. Three sections matter more than the rest: symfony vs laravel performance ownership of the code, confidentiality, and vue.js development services termination and handover. All the work product has to transfer to you as it is paid for, together with documentation, pipelines and deployment scripts. Look closely at any clause that keeps reusable components in the vendor’s hands, as that is often the part you cannot replace later.
Find out how the estimate was built. A serious estimate is accompanied by a list of assumptions, a breakdown per feature and an explicit range. A fixed price is only reasonable when the specification is complete; otherwise the provider prices the risk in and you fund the buffer regardless. Hourly billing shifts that risk to you, so it requires visible weekly reporting and a spending cap.
Process beats headcount. Find out what happens when the scope changes, who writes the acceptance criteria and what the QA setup looks like. A team can walk you through a live build at the end of each sprint. Written acceptance criteria remain the practical protection against endless rounds of rework.
Before signing, think about the handover while the relationship is still good. Require that the code repository lives under your account from the first commit, and that documentation is updated as part of the work. A partner who is comfortable with this accepts it without argument; hesitation here says a great deal.