That feature requires internet access. In fact, just activating VEGAS (365 or perpetual) requires internet access.
Do I need to be constantly connected to use the STT feature? I mean, I can connect to register, etc. if I need to.
You would only need to be connected when using the tools that require internet access (the hub, talk to text, text to speech...) It will also need to be connected the first time you use some codecs. And, I think, about once a month? week? it also want's to "phone home" to see that you're still valid.