Vite server proxy is a http-proxy (https://www.npmjs.com/package/http-proxy). Therefor the same config settings can be applied.

export default defineConfig({
  base: './',
  server: {
    proxy: {
      '/data': {
        target: 'https://example.com/foo',
        changeOrigin: true,
        configure: (proxy, options) => {
          // proxy will be an instance of 'http-proxy'
          const username = 'username';
          const password = 'password';
          options.auth = `${username}:${password}`;
        },
      }
    },
  },
})