This follows some of the posts that I have seen on the Cloudflare Community page on "Custom domain with blogger not working".
I recommend setting up the redirect on Cloudflare.com, so that Cloudflare's edge locations will do the redirecting effort, rather than you needing the visitor to send a request back to your origin as part of the redirect work.
So here's a few things that I did to have this work.
1. Set IP addresses to the same per Google's support article
2. Set www. as CNAME to vulongtran.com
3. Set SSL to Full mode
Please try using the the SSL = Flexible mode if it is breaking, as we cannot upload an origin SSL to blogger.com.
4. Set a Page Rule to redirect
This is more optional to do at Cloudflare's edge rather than on Blogger.com's settings. As that'll set the redirect on the origin server, which means that the request needs to find your origin to complete the redirect.
I recommend setting up the redirect on Cloudflare.com, so that Cloudflare's edge locations will do the redirecting effort and work, rather than you needing the visitor to send a request back to your origin server to do this redirecting work.
Forwarding URL: 301 Permanent Redirect
5. Test that is working
So far it is working fine: https://www.vulongtran.com
Many thanks to Nico (Andronicus Riyono) from the Cloudflare Support team for sussing out the solution here and recommending the Page Rule level redirect!
Note - I brought a dedicated SSL certificate so it looks like this: