When we say vidhi-vākya and niśedha-vākya in respect of Brahman, we need to understand them correctly.
Vidhi-vākya are mahāvākya like tatvamasi, aham brahmāsmi, ayam ātmā brahm and prajñānam brahm.
Basically those descriptions which aver the identity of ātmā with Brahman is called vidhi-vākya.
Niśedha-vākya are those which merely illustrate the swarūpa of Brahman negatively. Like neti, neti, asthūlam, ahrasvam etc.
There is a proper discussion as to which one is primary and which one is secondary.
There are three views in this regard:
1. Vidhi-vākya is pradhāna. Niśedha-vākya are upakārī to vidhi-vākya as they purify the meaning of the words of vidhi-vākya like tat and tvam. The niśedha-vākya remove the impurity from tat and tvam (bhāga-tyāga) and arrive at unitary idetnity.
2. Vidhi-vākya are pradhāna and they imply niśedha-vākya. When I understand that I am Brahman, naturally it gets implied that I/Brahman am none of the seen. So, niśedha-vākya are a byproduct of vidhi-vākya. They can alternatively also be arrived at through logic. So, Śruti merely does the anuvāda of niśedha-vākya which results from vidhi-vākya.
3. Niśedha-vākya are either pradhāna or equally efficacious as vidhi-vākya. Through niśedha-vākya, when entire seen-world is negated, then mind automatically becomes of the form of Brahmākāra-vṛtti. And hence, the same result is arrived as that of vidhi-vākya.
This is discussed in Saṅkṣepa Śārīraka 1.250 to 1.258.
In my understanding, mukhya-sāmānādhikarṇya is amenable to #1 and #2 whereas bādha-sāmānādhikaraṇya holds #3.
Dṛṣṭi-sṛṣṭi-vāda would align with #3 while sṛṣṭa-dṛṣṭi-vāda will align with #1 and #2.
Regards,
Sudhanshu Shekhar.