First I will thank everyone who helps here. MiaB is a great tool and light weight I guess.
My approach is to run a mail server on different vps and run a web server other vps.
The problem is I have already running my web server and pointing the A record to the vps IP but I am not sure how can I run MiaB without touching that. I have little knowledge about the how the system works.
I agree with @openletter’s approach, but that may not be the approach that you wish to take.
If you are running a webserver elsewhere, I highly recommend that MiaB not be utilized for the DNS services of the domain. That said, you will need to skip some steps in the setup and will need to copy some DNS records over to your existing DNS provider.
Specifically, you DO NOT create glue records for your domain. You DO add a A record in DNS for the subdomain box (assuming that you are following the naming in the guide - you can name it something else if you wish) pointing to the VPS that will be running MiaB.
Set up your MiaB server per the instructions. When complete there will be errors on the system status page concerning DNS which you can ignore. You will need to copy the records listed on the External DNS page to your DNS provider.