Cannot POST php mail (Vue) - php

Question

I am building a really small webapp in Vue with a simple contact form as component. I would like to use a really basic php script for sending the mail. I am not an expert on PHP but for some reason I can't figure out this bug I get when I try to submit the contact form. It says
Cannot POST /mail_contact.php
HTML/Vue component
<form class="contact" action="mail_contact.php" method="post">
<div class="container">
<input type="text" class="st-input" placeholder="Vul hier jouw naam in" name="name">
<input type="text" class="st-input" placeholder="Vul hier kort en bondig jouw probleem in" name="problem">
<input type="text" class="st-input" name="email" placeholder="Vul hier jouw e-mail adres in">
<input type="text" class="st-input" placeholder="Vul hier jouw telefoonnummer in" name="tel">
<select name="telOrEmail" required><option disabled>Maak een keuze</option><option value="Tel">Telefoon</option><option value="Email">Email</option></select>
</div>
<button class="send-button" type="submit">Verzenden!</button>
</form>
PHP Script
<?php
if(isset($_POST['submit'])) {
//getters from form
$name = $_POST['name'];
$tel = $_POST['tel'];
$email = $_POST['email'];
$problem = $_POST['problem'];
$telOrEmail = $_POST['telOrEmail'];
//mail content
$formcontent=" Van: $name \n Telefoonnummer: $tel \n Emailadres: $email \n Problem: $problem \n Terugbellen of Emailen: $telOrEmail";
//ontvanger van mail
$recipient = "g.gijsberts#sqits.nl";
//Onderwerp
$subject = "Contact formulier Sqits-it";
$mailheader = "From: $email \r\n";
mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");
echo "Bedankt voor je bericht! Wij hopen zo snel mogelijk te reageren.";
}
?>
Is there something I just don't see?
Edit
Project structure
index.html (File)
src (Directory) -> App.vue, main.js, router.js, components (directory) -> Contact.vue
dist (Directory) -> build.js, build.js.map, index.html, mail_contact.php
mail_contact.php (File)
So as you can see I've place my php script in the same directory as the main file (Index.html), but also in the directory where the build file is placed. Good point to say is this is a Vue project.
For a weird reason I get a 404 error (on localhost:8080/mail_contact.php) when submitting the form before I am redirected to the blank page with the Cannot POST /mail_contact.php. How is this possible if the file is actually there in my directory?