Do I need to change my OBS setup to stream on both Twitch and VERGR?

No. Keep streaming to Twitch exactly as you do now. Vergr picks up your Twitch stream by itself once Twitch is connected.

You send one stream, to Twitch, so your upload doesn't double.

Prefer to stream to Vergr and send it on to Twitch? Choose OBS / RTMP on the Go Live screen and turn on Stream to both. That one does use Vergr's stream key in OBS.