By default, Visual Studio Code will try to guess your indentation options depending on the file you open.

You can turn off indentation guessing via "editor.detectIndentation": false.

You can customize this easily via these three settings for Windows in menu File → Preferences → Settings or Ctrl+, and for Mac in menu Code → Preferences → Settings or ⌘+,:

// The number of spaces a tab is equal to. This setting is overridden
// based on the file contents when `editor.detectIndentation` is true.
"editor.tabSize": 4,

// Insert spaces when pressing Tab. This setting is overriden
// based on the file contents when `editor.detectIndentation` is true.
"editor.insertSpaces": true,

// When opening a file, `editor.tabSize` and `editor.insertSpaces`
// will be detected based on the file contents. Set to false to keep
// the values you've explicitly set, above.
"editor.detectIndentation": false