Ah okay, so if you were using v1 previously, installing any v2 templates (such as Vengeance) would have resulted in your website breaking .
Now you are using v2, you mention the template doesn't appear on the list any more. This is probably because the template is outdated (for example, v2 pre-release 4 templates such as Vengeance will not install on v2 pre-release 5).
If you take a look at the templates Resources section at
https://namelessmc.com/resources/category/2-namelessmc-v2-templates , any templates from Rapid and up should be updated to pre-release 5 currently.