Question: How can anyone include a Vedic text that describes the Glories of Lord Krishna, like the Brahmavaivarta Purana, as being in the rajo-guna (mode of passion). Could you please explain?
Thank you for writing. The Brahma-vaivarta Purana is overall categorized as being a rajo-guna Purana, aimed at elevating those within the mode of passion. The pastimes of Lord Krishna within this Purana are specifically there to benefit those souls who are attracted to lower modes.
Descriptions of those pastimes are not within the lower modes. It is like the Bhagavad Gita occuring within the Mahabharata. Though the Mahabharata is focussed on dharma (religiousity), artha (economic development), kama (sense enjoyment) and moksha (liberation form suffering), the Gita itself is not interested in any of these, but instead focusses on complete surrender to Lord Krishna.
The entire Vedic literatures are presented in this way. Certain portions appear to be offering lower things to the conditioned soul, but they are presented (by Vyasa) in such a way that one will be exposed to Lord Krishna and His spiritual pastimes, and in this way the soul is purified from his lower desires and eventually comes to the platform of pure devotional service, ananya-bhakti.