Hi
You should have the following agreements with your vendor in order to ensure that your idea and efforts on development of tool are not vandalised by the software developer and /or marketing agency.
1) Intellectual property agreement clearly outlining the fact that all IP, copyright, trademark will remain with you and your vendor will have no rights whatsover on your tool and its derivatives.
2) A non disclosure agreement cum indemnity and
3) Software development agreement (for development of tool/online website tool) between you and software developer outlining the scope of project, deliverables, timelines and also handing over of source code to you.
4) Commercialisation agreement (assuming the developer will take responsibility for marketing and sales of your tool.
If you believe that your tool is commercially viable, you should hire the services of a lawyer who can be of assistance in drafting the relevant contracts. You can hire lawyer on assignment basis (time and material) or on per contract (each of above mentioned clauses 1 to 4) is a contract.
Many of us lawyers work with start up's and we are aware of the challenges of a start up and we provide services keeping in mind the challenges of start up
Hope this information is useful