With a vanity domain, you can fully control the TTL values. All my sites use a vanity domain because it doesn't tie me to a particular CDN, and they have 86400 TTL.
When you have a vanity domain like cdn.example.com, the recursive resolver already knows the nameservers for example.com, so this actually reduces the additional DNS lookups.
When you have a vanity domain like cdn.example.com, the recursive resolver already knows the nameservers for example.com, so this actually reduces the additional DNS lookups.