When vaccination is done through drinking water, it is a good practice to mix a dye in the water to check after the vaccination if all birds (or the majority of the birds) did get to drink the water with the vaccin.
However, the effectiveness of the vaccination does not only depend on the fact if all birds drink the water, but also at which moment after the mixing of the vaccin in the water they did drink it. Life vaccins have a limited life span, which largely depends on the composition (chemical balance and additives) and the temperature of the water. This means that there is a strict time frame after preparing the water in which the birds should drink. Birds that drink the vaccin after that time frame will have a limited chance to get vaccinated, even if the dye has coloured their tongue.
So its a good practice to check the colouring of the tongues at the moment the theoretical life span of the vaccins has passed (which can be estimated from the temperature of the water etc), or to change at that moment the water with vaccin and dye for clear water, and check the tongues at a later moment. If it shows that at that moment not all the birds did get the opportunity to drink, the vaccination procedures should be evaluated to increase the effectiveness.