I'm a new simmer, just got TS3 and a few expansions last week, and I've always been fascinated by vampires and other supernatural beings, so the first expansion I bought was supernatural. My sim's girlfriend is a Vampire, and I think he'd be perfectly fine with letting her drink from him, in fact the main reason I romanced a vampire in the first place is because I thought it would be sweet and romantic if he trusted her enough to drink from him. I was hoping for an interaction that lets her know it's okay, but I haven't seen one. I would actually like her to default to drinking from him when she's thirsty instead of grabbing a plasma juice. She's never even tried to drink from him before, but as I said he'd be totally fine with it. Is it possible in the vanilla game to do this, or is there a mod that adds this in?

Is the vampire playable / part of the playable family or not?

Well, I couldn't remember if drinking from a Sim is coded to be autonomous so I sifted through my mod folders, opened S3PE and looked through the GameplayData's ITUNs and Social XMLs and for the life of me cannot even find reference to that action, so I've either overlooked it or it's not obviously named. But I'm guessing that it is autonomous by default being that I haven't changed the coding to suit my More Autonomy mod. So the option is most likely already in-game but it may only be available for your Sim's gf. I don't think Sims have the option to offer their blood to a vampire, only to ask to be turned. However, in case I'm wrong and it isn't autonomous, Bluegenjutsu has a mod here that gives vampires more freedom all around. You will need to register with the site to access her mods. And, don't worry, there are different "flavors" of her mod in case, say, you don't want vamps to auto-turn other Sims or something.

UPDATE: Found it! It was hiding in the base game Social Data where I wouldn't have thought to look. EA's weird like that. Anyway, yes, drinking is autonomous so you shouldn't need Blue's mod for that, only for the other vamp interactions. However, drinking appears to be a romantic social, so if their relationship is not high enough yet, she may not be able to drink from him. It appears the relationship value for a successful interaction is set at "73" (if I'm reading it right) and the receiver needs to at least be a romantic interest. However, the interaction is also coded as "friendly" and that value is set at between "71" and "74" depending on circumstance, so romantic status may not be necessary as long as the relationship is high enough. (Sorry, I don't have enough experience with vamps in-game to have properly tested that theory.)

@nikel23 I haven't asked her to move in so I don't have control of her yet. I suppose if i did move her in i could just use the drink interaction with her, but I'm an avid roleplayer at heart and I have decided my sim would do anything for her, so I'd like to have him offer his blood to her himself. (I know that might seem silly, but I do things for roleplaying purposes in all sorts of games that border on tedium in some people's minds, just ask my D&D group XD)
@chreai Thank you so much for looking into this. I had a feeling there wasn't a way to have a sim offer their blood, but that's what I was hoping for. As I said the vampire is his girlfriend, so already past the romantic interest stage, and the green bar under her portrait is comepletely full, so it's at whatever the maximum value is. I guess I'll just ask her to move in and manually tell her to feed on him when she's thirsty, that seems to be the best solution right now. Thanks for your help! :D

Mr. Tucman,

Your approach sounds right. to me. I have also tried to do everything possible within the game & by means of free will. Yet I think it is proper for a player, who is aware of the yearnings and the practical limitations of their Sims, to help them work around those limitations and realize their destiny.
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You're welcome!