Thanks for pointing that out. In Vivaldi installer there is an "Advanced" button and clicking on it gives you the option to choose installation path. The default installs Vivaldi in the current user directory, but if you tick install for all users, then it will install in Program files.
Correct. You need to run the installer as admin in order to do this though. I agree with @oldschool; installing Vivaldi in \Program Files\ would be more secure because of how Windows 10+ locks that directory down. That's where I have mine installed.