Yeah, the problem here is that the rules are unintuitive. You create a new land type that is the name and land type of a new basic land and you want that land type not to be a basic land type despite doing everything in your power to create the impression that it is.

Creating this card without making Vacuum a basic land type outright removes any intuitive implicit way for players to define what a basic land type is.

As a custom card designer it's better to bite the bullet and create a basic land type and rely on the fact that you don't interact with the full card pool than creating a solution that realistically cannot be chosen. Either way you are going down a path WotC is avoiding and at least just creating a new basic land type is something they might do by simply abandoning the old cards towards a wording of "Choose a basic land type other than Vaccum" etc.

For reference on how confusing Vacuum actually will be: I still regularlymeet people who think Waste is a basic land type despite Waste not even being a land type.

Honestly it would be better to create a new wording and rules for effects like Convincing Image than the other way around - and the interaction with a sixth basic land type is not necessarily a problem even on most cards that let you make a choice.