My experience on Ubuntu 16 was that mysql was installed by default. This caused problems, as mysql was version 5.7, and mariadb 10.x corresponds to mysql 5.6. There are substantial differences in the table structure between 5.6 and 5.7, so effectively downgrading from mysql 5.7 to mariadb 10.1 was a lot of hassle.
I think your best approach is to manually install mariadb first, and then run the vesta install script, with the "no mysql, no postgresql" option.
And to answer your question 'when', its been proposed as a feature, but not yet accepted. https://github.com/serghey-rodin/vesta/issues/1204