Here's why:
* Van is already a shortened form: It's the short form of "caravan" or "covered wagon".
* Diminutives often have a cute or endearing connotation: A diminutive like "vannie" would sound somewhat childish or informal.
If you want to convey a smaller size or a sense of affection, you might use phrases like:
* Little van
* Tiny van
* Compact van
* My trusty van
It's important to consider the context and your intended tone when choosing a term!