For the topic of Dharma the Vaisheshika Sutras are considering word of vedas as reliable source of knowledge. It's said in the below sutra that the Ishwara created this universe through the Dharma of Dravyas (knowledge of elements) . And the information how Ishwara created this universe through the Dravyas (elements) is found in the Vedas. So on this basis of this knowledge of (elements) i.e how ishwara created the universe through Dravya Padartha from vedas ,the Words of vedas are considered as Valid Source of knowledge according to Vaisheshika Philosophy. Vaisheshika accepts that the vedas as word of God.
Its also said in the sutra that Veda-Bhagvan i.e. Ishwara in the form of vedas ,know each and everything of this universe. On the account of description found in the vedas about the creation of universe , the vedas are considered as Valid source in all subjects. The Real nature of Dharma is only described in Vedas. the fruitfulness of performing of Yajnas , sacrifices are also observed , which is described in the vedas. i.e result of performing Yajnas.
Vaisheshika Darshana Says that the seers of Vedas i.e. the Rishis were highest sadhakas , they were Tri-kal Darshi's , they were very well aware of Past and Present. Because they were expressing their opinions by through research , that is why the mantras heard by them is Swa-Pramana. Because they were doing austerities. And Considering these qualities of the Seers of Vedas and these seers heard the Mantras of vedas i.e. divine words ,their words are to be considered as Pramana or valid source of knowledge. Its the divine words heard by seers and because of the the qualification of those seers also they are considered as valid Pramana. So vaisheshika Sutras Consider words of vedas as reliable source of knowledge or the Shabda Pramana because of Rishi's.
Because the vedas always speak about truth . And those seers always thoughr about welfare of other beings the divine words heard by them is considered by vaisheshika as Shabda Pramana
.( Here the word Dharma means the knowledge of Padarthas or elements).
This is explained in Vaisheshika Sutras Chapter 1- Anuvaka 1 - Sutra 3.
तद्वचनादान्मायस्य प्रामाण्यम् ||3||
The authoritativeness of veda (arises from its) being the Word of God
[ or being an exposition of dharma]
Here we can read the sutra in Hindi
The same can be found in the last sutra of vaisheshika Darshana.
Vaisheshika SutrasChapter 10 - Anuvaka 2 - Sutra 10
तद्वचनादान्मायस्य प्रामाण्यमिति ||
So on the basis of all these points Vaisheshika is accepting the words of vedas as Valid source of Knowledge.