Usually, you will get the error "Error: Let's Encrypt validation status 400" when installing the Let's Encrypt certificate if your domain does not point to the correct IP address. Let's say your domain points to IP 'a1.a2.a3.a4' in the control panel and to IP 'b1.b2.b3.b4' in the DNS. MyVestaCP will throw this error in this case.
To fix the error, you need to change the IP address in the DNS. Both DNS and the control panel should be set to the same IP address for a given domain. Once you change the IP address in the DNS, you need to wait for some time for the DNS propagation so that the new IP is reflected for your domain.
Once DNS propagation is complete, you can try to re-install the Let's Encrypt certificate, and it will not give any error.