From what we have in the both novel series, it seems to me that they lose most of the human emotional capability right upon turning (they see the beauty of nature yet feel nothing, and it’s explicitly said so in at least 3 cases so far: Crowley’s, Mika’s and Mahiru’s), but truly unfeeling they become with time, when the memories of how emotions used to feel dim and when they lose any emotional connection with humans (again, take Crowley: even after a few decades of being a vampire, he still cared about Jose; even if the rest of the humans started looking like food to him, Jose apparently didn’t; and only when that last bond fell away in a very painful way with Jose’s death, did Crowley truly give in to being a vampire). It’s like Ferid said: only important stuff can be remembered, so for feelings to remain to some decent extent, vampires would need constantly renewed bonds, I believe. This is their main problem, imo: the old ones fall apart with time, simply because humans die with time inevitably, but new ones are very hard to form for vampires, not just because of being emotionally stunted, but because they don’t really try, what with seeing humans as food and an inferior race and all that jazz. Still, it seems to me that a. bonds from before being turned apparently remain to some extent (Lacus and Rene, and Krul and her brother), and b. forming new bonds and preserving at least some of the emotional capacity is not entirely impossible (I base this on Ferid’s words about vampires giving their blood to the humans they take a liking to, that is, they’re at least capable of taking a liking to someone; also, Krul apparently came to value Mika not only for him being a dormant Seraph), just very hard and requires their active effort and hard work on the mindset, which is what is usually missing.