By default, when you split your current terminal vertically, the new pane will open a new session based on the default profile (e.g., PowerShell), which may not be what you want. For example, your current terminal is a WSL session, and you want to split the terminal into two, both of which are using WSL sessions. To override this, type CTRL+, to open your terminals settings, and click Open JSON file, located at the lower left corner. Add this item in the actions array.
{
"command": { "action": "splitPane", "split": "vertical", "splitMode": "duplicate" },
"keys": "alt+shift+plus"
}