TL;DR: There is no domain name ever provided with a VPS by default, in most VPS providers' setups, as it's likely your provider (and a lot of other VPS providers) are not Domain Name Registrars and do not provide the option for you to purchase a domain name to go with your server.
Slightly more informational:
There is no real need to assign a 'domain' to a VPS. Therefore, VPSes don't typically come with a domain name. I believe you are misunderstanding how VPSes work from providers.
In some cases, VPS providers also provide you with the option to register a domain name for use with the VPS. However, most VPS providers are not web domain registrars in their own right, and therefore they do not provide you a domain.
You have to purchase a domain separately through a registrar, and point the domain name's DNS
A record to the IP address of your VPS. From there, you can then set up a web server to listen and serve content for that domain. Note that this is a separate service from a separate company, and you need to set up the domain and DNS in order for it to work.
IF you have no domain but want your sent-locally emails to have something at the end like
email@example.com, you can try and set your FQDN with
sudo hostname hostname.com or something, but this is still not the answer to 'What is the default domain', as there is no default domain.