RH has a blog series which motivates and introduces vDPA.
For a start, read their post on virtio-networking and vhost-net [0], continue with virtio-networking and DPDK [1] and finally read "Achieving network wirespeed in an open standard manner: introducing vDPA" [2].
In particular the last link has a lot of diagrams, recaps a lot of the virtio-networking topics and finally has a table which compares the other solutions to vDPA.
* Introduction to virtio-networking and vhost-net [0] * How vhost-user came into being: Virtio-networking and DPDK [1]
There are more articles but these help with understanding the motivation behind vDPA.
[0] https://www.redhat.com/en/blog/introduction-virtio-networkin... [1] https://www.redhat.com/en/blog/how-vhost-user-came-being-vir...