The difference between the Vedas and the Purāṇas is like that between the brāhmaṇas and the parivrājakas. The brāhmaṇas are meant to administer some fruitive sacrifices mentioned in the Vedas, but the parivrājakācāryas, or learned preachers, are meant to disseminate transcendental knowledge to one and all. As such, the parivrājakācāryas are not always expert in pronouncing the Vedic mantras, which are practiced systematically by accent and meter by the brāhmaṇas who are meant for administering Vedic rites. Yet it should not be considered that the brāhmaṇas are more important than the itinerant preachers. They are one and different simultaneously because they are meant for the same end, in different ways.
There is no difference also between the Vedic mantras and what is explained in the Purāṇas and Itihāsa. According to Śrīla Jīva Gosvāmī, it is mentioned in the Mādhyandina-śruti that all the Vedas, namely the Sāma, Atharva, Ṛg, Yajur, Purāṇas, Itihāsas, Upaniṣads, etc., are emanations from the breathing of the Supreme Being. The only difference is that the Vedic mantras are mostly begun with praṇava oṁkāra, and it requires some training to practice the metric pronunciation of the Vedic mantras. But that does not mean that Śrīmad-Bhāgavatam is of less importance than the Vedic mantras. On the contrary, it is the ripened fruit of all the Vedas, as stated before. Besides that, the most perfectly liberated soul, Śrīla Śukadeva Gosvāmī, is absorbed in the studies of the Bhāgavatam, although he is already self-realized. Śrīla Sūta Gosvāmī is following his footsteps, and therefore his position is not the least less important because he was not expert in chanting Vedic mantras with metric pronunciation, which depends more on practice than actual realization. Realization is more important than parrotlike chanting.