Both Vishnudharma and Vishnudharmottara are Upa Puranas but they are very significant and famous in the Vaishnava cult. Both the Puranas are well-known among the Vaishnavites as they have rules & regulations, rituals and duties related to Vaishnava cult and philosophy.
As a part of Vishnu Purana
It is said in the Vishnudharmottara Purana that it is the second half of Vishnudharma Purana. And both, Vishnudharma and Vishnudharmottara Puranas are the second part of Vishnu Purana. Vishnudharma and Vishudharmottara Puranas describes Dharma of Vaishnavas, worship pattern of Vishnu, Vishnava philosophy just as Vishnu Purana. It is claimed that both the Puranas were a part of Vishnu Purana. May be Vishnu Purana is 1st Part of main Vishnu Purana and Vishnudharma & Visnudharmottara are the second part of same Samhita. But the detailed study has not yet been done.
Form of the Literature
The form of both Puranas is not like Purana literature but like a shastra, scientific literature. Both of them were initially considered as Shastras but when Puranas of other sampradays (cults) arose then these two Puranas were taken as Purana Literature.
200 – 300 AD is the period considered for Vishnudharma Purana. During this period there was spread of Buddhism which was not acceptable to the author of Vishnudharma Purana. Hence he refers to people who were the followers of Buddhism in that period as anti religion, orthodox, uncultured, etc. Hence this Purana was composed to reconstruct the Vedic religion. May be Vishnudarma Purana was composed in northwest part of India because it has descriptions of that region.
Dwaitadweda philosophy is the overall philosophy of Vishnudharma Purana. Union of self with Narayana is considered as the ultimate aim of life. For that union with Parabrahma Dnyanayoga is suggested. On the other hand, worship of Vishnu is suggested to those for whom Dnyanayoga is not possible to attain. ओम् नमो वासुदेवाय-Om namo vasudevaya is the chant of this Purana.
Vishnudharmottara Purana is divided into three Khandas, main divisions. These three divisions have 269, 183 and 355 chapters accordingly. 5th century A. D. is generally the accepted period of this Purana. Dr. Hajara has suggested Kashmir or Punjab as the area for the composition of this Purana. This is a very rare example of a Purana where there are less laterday additions. This Purana is narrated by Sage Markandeya to Vajra the grandson of Krishna.
Acharadharma (rules and regulation) of Vishnavas is given in the Vishnudharmottara Purana, but those are not given by Bhagavata cult. It is given by Pancharatra cult. Bhagavata and Pancharatra are the sub cults of Vaishnavism. Worship of Vishnu which is suggested by this Purana is according to Pancharatra style. Though there are more number of verses which describes worship of Vishnu, there is no touch of tantrism to them.
Vishnudharmottara Purana has varied content such as origin of universe, description of earth, ganga river, stories of Vamana and Parashurama, Grahajyotishya, Phalajyotishya, description of sky, worship of cow, science of war, Dhanurveda. Another important aspect of Vishudharmottara Purana is – it has the origin of various arts. Vishnu danced on the great ocean after killing demons namely Madhu and Kaitabha, hence art of Dance originated. Narayana drew a picture of Urvashi from which the art of painting originated. Later the origin of the art of sculpture is considered from the art of painting. This Purana is an encyclopedia of culture and art. It is said that Chitra and Nritya are basic arts and one should not even think about other arts without considering these two.
Shankarageeta is another important part of this Purana. It is narrated by Shiva to Parashurama. Shiva has narrated the importance of Narayana in it and also reveals that he himself worships Vishnu.