Touhou Game Club

You're saying it as if Vampire Contract determine how all youkai are supposed to live their lives. Vampire is certainly one-of-a-kind race that is notably superior than most, but they don't represent anything but their own. The contract was made because Vampires had/have enough power to disturb the balance of Gensokyo.

The population of humans in Gensokyo is part of said balance.

One thing we need to get straight first is the fact that youkai must be 'making contact' with humans in order to survive, be it in the sense of killing, attacking, surprising, helping, or etc. The rule of thumb is that forgotten youkai are cease to exist, even in Gensokyo. Identity for Youkai represents their own life as an entity. Without humans, youkai cannot exist.

Just like what Shikieiki said in PoFV, youkai are supposed to attack humans and humans with abilities are supposed to repel them.

Although there are still classic youkai, such as Tengu and Kappa, both of which are also referred as youkai that have developed their own characters, thus allowing them to exist even without preying on humans. One notable example of specific characteristic is how Aya referred to their fellow tengu as "It's different, we're only rumormongers nowadays" in Bohemian Archive in Japanese Red.

Gensokyo is a place that welcomes the forgotten.
But even without such label, it's still a place that youkai would call an Utopia. Disturbing the balance of it is equal to that of mass destruction.