Vault is a normal AppVM, not template. Also, it’s not based on a minimal template, but on a standard one. Minimal templates are an advanced feature for experienced users. If you can, it’s recommended to use them.
This is the same.
It’s the question of whom you trust more. Whonix is based on Debian, so you have to trust both. It’s better to trust less people, not more (i.e. Debian is probably better than Whonix). A better idea is to use a minimal template.