Vowel harmony is a phenomenon in some languages whereby all vowels in a word must be members of the same subclass. A language typically has two subclasses of vowels based on some kind of feature, for example: the language's vowels could be divided into high and low vowels; if a word begins with a high vowel all other vowels must be high vowels.
Vowels may otherwise be grouped into front and back vowels, rounded and unrounded vowels, or nasalised and unnasalised. Languages may have three groups. In such cases it is often that the third group is a set of neutral vowels that can be appear in either of the other two groups. Vowel harmony is also reflected within morphemes and by alternations in vowels when morphemes are combined into complex words, sometimes creating allomorphic alternations.
In Vulgar, you are able to create two groups of vowels, and each word will be generated using vowels from one or the other groups. If you want to simulate neutral vowels, you can put some vowels in both groups.
Be aware that Vulgar currently does not simulate vowel harmony in morphemes. For example, a root word like ‘violent’ may be generated with vowel group A, and the affix to turn adjectives to nouns may be generated with group B. Thus ‘violence’ will be a combination of ‘violent’ (group A) We hope to make implement some smarter assimilation in future updates. But simulating real language is pretty complex and Vulgar makes great progress in small steps.