With AirDroid + withExEditor
First, you establish a connection between a laptop or desktop computer with a proper keyboard and the Android phone.
On the phone, install the AirDroid app.
On the laptop, use the web application to connect to your phone. Currently (2018-06-21), this requires a free registration.
Using "Messages" in the web application (see screenshot below) you'll be able to write text messages on your laptop, sending them via your phone.
To write those messsages conveniently using Vim, you can use the withExEditor add-on for Firefox and Chrome.
After installling the add-on, and doing some additional setup on your laptop, you can use gVim to write your text messages.