I'm a little confused as to why nameservers are involved. If you're trying to setup email, you don't need to fiddle with the name server settings. Normally you can use HelioHost's name servers, but they're currently impacted by the revocation of our cPanel licenses. I'm not sure who your domain provider is, but for example my domain came with preconfigured name servers. If you're trying to point the domain "zaldivar.mx" to your VPS, you'll want to create an "A" record that points to your VPS's IP address.
For email, you'll want to create an MX record that points to your email server. It's a little complicated - the MX record needs to point to your email server, not an IP address. Mine is configured to point mydomain.com to mail.mydomain.com. The second domain (mail.mydomain.com) can then point to your VPS's IP. However, I don't think that's the cause of the "Service Unavailable" page you're seeing - for that I'd check the error logs, if Hestia provides them.