The vampire had sex with Ian in the earlier chapters as well. ( Around the time Ken went back the first time). Ian even mentioned how he is mad that the vampire let him go away after they had sex. From the earlier chapters it seems like both Ian and the vampire like Ken. I do find it weird that they are willing to share him I guess but I wasn't surprised when the vampire came in chapter 14 and was saying he loved him despite already having Ian as a sex friend/ partner. I am also not surprised he goes and had fun with Ian right after. He just seems like that type of dude that only wants to have fun even if he " loves someone".
Ok mentioned isn't the right word but it still seems pretty implied that they did. If you check again they aren't exactly wearing clothing. You can see Ian's shoulder and neck so he at least doesn't have a shirt. The vampire isn't wearing his shirt right either and doesn't have pants. There's a point when he moves that you see the lower part of his hips and part of his thigh. Also his shirt used to be buttoned and on properly but now he is using it more like a cape ( to cover his missing arm I assume). I also thought the way he bushed his hair, sat him in on his lap ( even if it was to get his blood it could have been done in a less seductive/ suggestive way), removed his hair tie and even woke up next to each other the next day seemed to kinky and suggestive to be a normal vampire and thrall relationship.

Okay spoiler.
Spolier alert
I look the raws and in chapter 18 the vampire was having sex with his thrall ian. I got really furious. Just moments ago he was touching ken and confessing how he loves him in ch14 . He put kein under spell so it was practically rape but I tolerated it because I thought vampire loved him before and kein doesn't remember it. Now I know that this motherf***rr vampire is two timing ken and ian... good looks of the vampire really is wasted away by his two timing personality...you can't just confess love to someone then go around and fuck someone else...ew
However I doubt something...what if the confession of love to kein was actually ian's part of speech??? I mean vampire was in body of ian , his thrall. What if vampire want Kain for other purposes what if it's ian who actually has romantic purposes towards kain ???
SOMEBODY SPOIL IT TO ME.