TL;DR: I have developed an integration for Home Assistant to talk to Victron inverters / chargers using the Victron MK3 USB interface.
Source code and documentation: GitHub - j9brown/victron-mk3-hacs
Why make a new Victron integration for Home Assistant?
There are already a few integrations for monitoring Victron devices from Home Assistant and they each rely on different interfaces and have different capabilities.
Until recently, I used a Victron VE.Bus Smart Dongle to monitor my Victron Multiplus II over Bluetooth LE and I controlled the device state using an optocoupler wired to the remote on/off terminal. It worked well but I could only switch the device on or off and I wanted to programmatically switch it between on and charger-only instead.
It turns out the Victron MK3 USB interface can set the operating mode and current limit programmatically, thereby achieving my goal of toggling charger-only mode. It also exposes a little more detail about the device’s behavior such as the AC input and output voltages and frequency which is nice. I’m not aware of any existing Home Assistant integrations for the Victron MK3 USB interface so I just wrote one. 
I hope you find this new integration useful and I welcome your feedback!
Jeff.