Hello,
This is another topic we discuss on our official website. If you would like to change the default subject of customer email message when a new reservation is placed, then you should override the default Gettext language string.
You are free to do it by using any third party plugins available, or you can change it directly in the PO/MO files. For sure, the best solution is to provide an override for the default translation string.
In any case, this is something that could not be changed through our plugin VikRentCar, unless you hard-coded the default PO/MO language files, and we obviously do not recommend doing that.
We hope this helps!
The VikWP Team
