I don't remember when the Webservices - REST object was introduced, but it should be available in V5. Even if it's not, you can just downoad it from the Digital Exchange and import into a V5 environment. In my opinion though, that object isn't all that necessary. I would just call the Utility - HTTP VBO directly and only provide the inputs that you need to use. One reason I'd suggest this is that I find it often unnecessary to do collection to json conversion when dealing with web services. It of course depends on what kind of data you're passing, but you may save yourself a headache by going straight to that HTTP object. As Tetsuji says, you could also write code to do it but I'd try Utility - HTTP first.
------------------------------
Dave Morris
3Ci @ Southern Company
Atlanta, GA
------------------------------
Dave Morris, 3Ci at Southern Company