I'm running into a bit of an issue with my homebrew rogue subclass. 

The level 3 class feature has two class options, one of which gives the character the vicious mockery cantrip.  I have the character listed as a spellcaster, as well as Vicious Mockery listed as an "Additional Specific Spell" on the basic page. 

For whatever reason, when I test out the functionality, Vicious Mockery's damage is not scaling with the character level. When I have them set to Lvl 20, it still shows as 1d4. I did nothing to override any aspects of the spell other than making the sure Save DC was the modifier I wanted it to be, and that the they can cast the spell starting at level 3.

Because it's on a class feature option, there is no level scaling ability specific to that option, so I'm not sure where to go from here to fix the scaling issue. Any help would be greatly appreciated.

By setting it to cast at 3rd level, you locked it into being cast as if the caster is always 3rd level.  If the feature is a 3rd level feature, that’s all you need.

Great, thank you!