In halacha, there is a concept of sticking with the hechsher of the city, and not bringing in another. I am not sure how that plays out in areas with several kehillas, each with their own standards, but I do know that it is not so simple to bring in a new hashgacha to an area that already has one, or to rely on it once it comes in - especially if the standards are essentially the same.
There is no basis in Halacha at all to not allow a second hashgacha in town. In fact, there are already places with other hashgachos already. But their standards are lower and the Vadd did not view them as a threat.
It would be more of a halachic issue to open a shul in the territory of another shul. And that nobody cares about!! The same we we need different schools and different schools with different hashkofos, we can also have different Vaad's and their hashgochos. There is no basis in Halacha prohibiting that.
The current Vaad is actually a merger of two different Vaadim years ago. The same way there was nothing wrong then when the Vaad originally opened up, certainly now as the area has grown we can have multiple hashgochos.
Everyone would understand that if Belz would open a kehila nearby, they would have their own hashgocho with their own standards. Most people here would not even want them to utilize the current Vaad by upgrading their standard to a Chsideshe hashgacha. In our circles it's the same way. There are different standards and different hashkafos.
I am not saying I agree with the new Mehadrin's standards. But the Vaad has no halachic basis to tell people not to patronize these establishments.