Disclaimer: I havent used ether of the 3 so my reply is only based on what I have read and heard

Vapor is a different way of hosting your solution is is a serverless solution which means you don't have to manage the servers/infrastructure and allows the application to scale automatically when needed without human interaction. the 40 a month is only the cost of using vapor you will still have to pay for what you use on amazon

Forge is a system to manage servers for you through an easy to use gui it will setup the server and manage it but you are responsible for manually scaling the servers and connecting loadbalancers and other servers

Envoyer handles easy deployments for you and monitors your applications health

In my opinion vapor is the way to go as it reduces the amount of time you ave to spend on managing servers and infrastructure and will automatically spin up a new instance if one were to fail.

there are some minor drawback with the serverless approach on AWS such as the maximum project file size or the maximum execution time of 15 minutes but as long as you keep this in mind when developing the project and maybe splitting up the services into microservices will allow you a very flexible solution

I know that I cant wait to get my vapor invite so I can play around with it