Hi, I know Im very late but hey better late than never
So imo Dettlaff should take it fairly easily if he's enraged(not bloodlusted).
The whole "Vilgefortz was stomping Geralt, Yen and Regis at the same time" is BS and heavily avoids whole context. At the beginning it was only Geralt and Yen who was heavily weakened by the months of tortures and being cut off magic(yet still she was able to stand her ground againt Vilgefortz). When Regis arrived Vilgefortz was magically holding Yen in the air, after slashing his face she fell on the ground and was incapable of fighting. Vilgefortz was shooting in rage in the air but Regis easily dodged all of them and recreated(don't know how to translate that word better) himself next to Geralt, after that Geralt tells him to watch out and Regis responds "Watch out? Me? That's not why I came here!"(Translated by me from polish), I don't think a clear minded Regis would say that. After that Regis easily speedblitzes Vilgefortz and holds him by the neck, he could literally end Vilge right here but his bloodlust blinded him, Vilgefortz used this and used spell created by him to melt Regis. Right then it's just Vilgefortz vs Geralt which the white wolf ultimatley wins.
Now let's go with some explainings as to why I think Dettlaff would win:
1. Vilgefortz is not fast enough to hit him. Dettlaff should overall be comparable to Regis in terms of speed and I have found no info wheter drinking blood besides drunking them also makes Higher vampires faster
2. Vilgefortz A.P. This is rather for overall just Vilgefortz than about this fight but from
Regis Journey story from gwent chapter 9 Sabrina tells Sorensen that the spell used on Regis by Vilgefortz activates only on contact with Vampire which means it doesn't scale to his overall A.P. especially that its pre made spell, now you can say that it's because the spell was incomplete but Sabrina states later that what she recreated should be enough since the spell was very costly and challenging and since there is no other info about this spell, I think it's fair to assume that it doesn't work or works worse on different beings
3. Geralt's growth in power. Now as we know Dettlaff does scale to BaW Geralt but even before the dlc, Geralt from W3 story imo should be enough to defeat Vilgefortz. This is ofc backed up by multiple feats, I will try to make it as short as possible:
3a. Despite what most people think, Geraly fully recovers his full strenght already in the prologue of Witcher 1 instead of the end of Witcher 2 or beginning of Witcher 3
3b. Azar Javed, one of the main villains of Witcher 1 is said to
be soon another Vlgefortz by Triss, Geralt ofc defeats him at the end and his master, Jacques de Aldersberg
3c. Eredin and the wild hunt are superior to Vilgefortz and other foes/allies of Geralt pre-Dettlaff, normally I would send links to proofs of it but the probelm is that there are way too many spreading across games and guidebooks and Idk how to send screenshots here. It's really a topic for another disscussion
So yea, Dettlaff should take it if he's enraged/not holding back, sheer speed difference is enough. However Vilgefortz does stand a chance if Dettlaff wants to toy with him or is drunk like Regis was in lady of the lake
