@CobaltVR, I looked into this and it seems like disconnecting the USB from the battery side is the safest option to turn off the adapter. It does have a "standby mode" in the sense that it uses the SteamVR proximity sensor idle and thus there is less transmission occuring since there is no video transmission - this does indeed lower the draw from the battery but is not a true "standby mode" in the more traditional sense.
I've flagged all of this feedback to the product team as it's all quite valid.
To reach Viveport's customer service team, email firstname.lastname@example.org
To connect with a live support agent for hardware support, go to www.vive.com/support, click on "contact us" in the top righthand corner, and then opt to speak with a Live Chat agent David R
Technical Specialist - San Francisco, CA; Monday-Friday