I want to start a SaaS or product based company and thinking of going with the .NET family of technologies, as I have been working on it for the past 3 years. Can anyone provide insights on the pros and cons of this approach? Would I be able to run modern JavaScript frameworks on top of it like React/Vue.js/Node.js?
You can totally make JavaScript and associated frameworks work with .Net. Anyway, you'll keep the client-server and different modules in the system loosely coupled. I don't see any issue with this. Write the backend using .NET and use JavaScript for the front end.
You should be totally fine with .NET. You would benefit immensely if you are planning to use some of the PAAS services on Microsoft Azure. Most Azure PAAS services have a first class support for .NET.
A small advantage of going with Javascript on server side would be that you get to use/learn only one language both on the frontend and the backend.