1. vMix FMLE is using Adobe FMLE, so you need to install it.
2. There is no difference in the way it works, but when you use Adobe FMLE you've got more flexibility in settings (i.e. framerate).
3. No, vMix will start the dosbox version of FMLE (FMLEcmd.exe) by it self.
You can also use FMLEcmd.exe in stead of vMix FMLE (located in Adobe's install folder) the streaming will start automatically when you start FMLEcmd.exe

When you use FFmpeg in stead of FMLE then you don't need to install Adobe FMLE.
Search this forum for 'FFmpeg' for the differences between FMLE and FFmpeg.

(my experiences of FFMpeg: slightly better quality, less cpu use)

Actually, I am quite certain that FMLECMD is included in the vmix installation so you don't have to install the standalone FMLE. FMLECMD uses a little less resources than the graphical version.

You have just as much flexibility with the vmix streamer as you do with standalone FMLE...

I might also point out just for clarity that its not running in "dosbox". Its running from the command line, just for anyone who might get those mixed up who sees this.

Not to turn this thread into something totally off topic but the "Dosbox" you are referring to is, in fact, the Command Line and it is the same thing you are seeing in the vmix output just in a different place. It isn't DOS. It isn't Dosbox (which is another program itself). The last version of Windows to include DOS was ME. I was just wanting to point that out for clarity is all as I know plenty of people come through here and other forums that will get all kinds of mixed up if this kind of info isn't clarified.