cakephpで簡易サーバーを立ち上げようとしたが、autoload.phpがない

cakephpを使ってアプリを開発しようとしています。サーバーはvagrantを使っており,OSはcentos7です。

[vagrant@localhost myapp]$ bin/cake server -H 192.168.33.10 -p 8000

で簡易サーバーを立ち上げようとしたところ、

PHP Warning:  require(/home/vagrant/cake1/myapp/vendor/autoload.php): failed to open stream: No such file or directory in /home/vagrant/cake1/myapp/bin/cake.php on line 5
PHP Fatal error:  require(): Failed opening required '/home/vagrant/cake1/myapp/vendor/autoload.php' (include_path='.:/usr/share/pear:/usr/share/php') in /home/vagrant/cake1/myapp/bin/cake.php on line 5

と出ました。vendorの中はemptyになっていました。

対処法を教えて欲しいです。よろしくお願いいたします。

【追記】
php composer.phar install した時のエラー

Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - cakephp/cakephp-codesniffer 3.0.0 requires squizlabs/php_codesniffer ^3.0.0 -> satisfiable by squizlabs/php_codesniffer[3.0.0, 3.0.1, 3.0.2, 3.1.0, 3.1.1, 3.2.0, 3.2.1, 3.2.2, 3.2.3].
    - cakephp/cakephp-codesniffer 3.0.1 requires squizlabs/php_codesniffer ^3.0.0 -> satisfiable by squizlabs/php_codesniffer[3.0.0, 3.0.1, 3.0.2, 3.1.0, 3.1.1, 3.2.0, 3.2.1, 3.2.2, 3.2.3].
    - cakephp/cakephp-codesniffer 3.0.2 requires squizlabs/php_codesniffer ~3.0.0 -> satisfiable by squizlabs/php_codesniffer[3.0.0, 3.0.1, 3.0.2].
    - cakephp/cakephp-codesniffer 3.0.3 requires squizlabs/php_codesniffer ^3.0.0 -> satisfiable by squizlabs/php_codesniffer[3.0.0, 3.0.1, 3.0.2, 3.1.0, 3.1.1, 3.2.0, 3.2.1, 3.2.2, 3.2.3].
    - squizlabs/php_codesniffer 3.2.3 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.2.2 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.2.1 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.2.0 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.1.1 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.1.0 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.0.2 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.0.1 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - squizlabs/php_codesniffer 3.0.0 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
    - Installation request for cakephp/cakephp-codesniffer ^3.0 -> satisfiable by cakephp/cakephp-codesniffer[3.0.0, 3.0.1, 3.0.2, 3.0.3].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php.ini
    - /etc/php.d/20-bz2.ini
    - /etc/php.d/20-calendar.ini
    - /etc/php.d/20-ctype.ini
    - /etc/php.d/20-curl.ini
    - /etc/php.d/20-exif.ini
    - /etc/php.d/20-fileinfo.ini
    - /etc/php.d/20-ftp.ini
    - /etc/php.d/20-gd.ini
    - /etc/php.d/20-gettext.ini
    - /etc/php.d/20-iconv.ini
    - /etc/php.d/20-intl.ini
    - /etc/php.d/20-json.ini
    - /etc/php.d/20-mbstring.ini
    - /etc/php.d/20-pdo.ini
    - /etc/php.d/20-phar.ini
    - /etc/php.d/20-sockets.ini
    - /etc/php.d/20-sqlite3.ini
    - /etc/php.d/20-tokenizer.ini
    - /etc/php.d/30-pdo_sqlite.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

phpのバージョンは 7.0.29で、composer.jsonの内容は

{
    "name": "cakephp/app",
    "description": "CakePHP skeleton app",
    "homepage": "https://cakephp.org",
    "type": "project",
    "license": "MIT",
    "require": {
        "php": ">=5.6",
        "cakephp/cakephp": "3.5.*",
        "cakephp/migrations": "^1.0",
        "cakephp/plugin-installer": "^1.0",
        "josegonzalez/dotenv": "2.*",
        "mobiledetect/mobiledetectlib": "2.*"
    },
    "require-dev": {
        "cakephp/bake": "^1.1",
        "cakephp/cakephp-codesniffer": "^3.0",
        "cakephp/debug_kit": "^3.2",
        "psy/psysh": "@stable"
    },
    "suggest": {
        "markstory/asset_compress": "An asset compression plugin which provides file concatenation and a flexible filter system for preprocessing and minification.",
        "dereuromark/cakephp-ide-helper": "After baking your code, this keeps your annotations in sync with the code evolving from there on for maximum IDE and PHPStan compatibility.",
        "phpunit/phpunit": "Allows automated tests to be run without system-wide install.",
        "cakephp/cakephp-codesniffer": "Allows to check the code against the coding standards used in CakePHP."
    },
    "autoload": {
        "psr-4": {
            "App\": "src/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "App\Test\": "tests/",
            "Cake\Test\": "vendor/cakephp/cakephp/tests/"
        }
    },
    "scripts": {
        "post-install-cmd": "App\Console\Installer::postInstall",
        "post-create-project-cmd": "App\Console\Installer::postInstall",
        "post-autoload-dump": "Cake\Composer\Installer\PluginInstaller::postAutoloadDump",
        "check": [
            "@test",
            "@cs-check"
        ],
        "cs-check": "phpcs --colors -p --standard=vendor/cakephp/cakephp-codesniffer/CakePHP ./src ./tests",
        "cs-fix": "phpcbf --colors --standard=vendor/cakephp/cakephp-codesniffer/CakePHP ./src ./tests",
        "test": "phpunit --colors=always"
    },
    "prefer-stable": true,
    "config": {
        "sort-packages": true
    }
}

です。