I’ll try to do better in explaining my ideas in more detail. 
Part of this idea came from Champions of Norrath on the Playstation 2, which is set in Everquest’s world. I vaguely remember the details, but from what I remember, there is a female vampire thrall who wants you to defeat her master. One thing she asks for is for a plant that will sate her need to feed temporarily, since she doesn’t want to actually go out and harm others. As I said, the effects are temporary, the plant is hard to find, and if I recall correctly, the person who eats it will get a resistance to its effects over time. Again, it has been years since I played it, so I don’t quite remember all the details.
Having San’layn in a similar situation provides a few things:
First, it gives a backstory to them and a reason they’d want to reach out to the Horde. It also gives them some sympathy to have the San’layn, who just became free from the Lich King, have to deal with resisting an insane frenzy of feeding on things.
Second, it could explain where most of the San’layn our, including our player character San’layn. Most of them could be hibernating somewhere while others seek to fix their frenzy urges while having limited supplies that have diminishing returns.
Third, assuming Kael’thas ever came back as a San’layn, it would play into his redemption to help his people. The San’layn were his followers in life, so it can work well for our characters to help the San’layn in general.
Fourth, it plays into how Blood Elves and Nightborne had addictions. They both can be Horde factions that are sympathetic to the San’layn, want to help them, and would speak for them joining the Horde. Blood Elves in particular would want to help their former kin, even if they were followers of Kael’thas before.
Fifth, it can help give San’layn more trust afterwards. We don’t know how much is know with Dreven’s actions. Curbing the idea that San’layn would just randomly turn on their allies in an insane lust for power and sustenance would go a long way in helping to gain trust again.
As stated before, the idea isn’t to cure the need to feed. It’s more giving a San’layn control over it. They were released from the Lich King’s power and have to deal with being both undead and vampiric, which I imagine would be very hard to suddenly deal with the urges that come with such a curse. I wouldn’t really call them blood junkies, but giving in to said urges could explain why some San’layn went mad with power or insane.
I imagine San’layn still feeding as needed. Seeing as Warlocks do a similar thing with spells to either enemy soldiers or woodland critters, I don’t really see it too much as an issue with the Horde. It just becomes something that is controlled and doesn’t really drive them insane (expect perhaps a battle frenzy racial).
The San’layn having vampiric traits is very important to their character. What I typed above is far from a perfect idea and needs fleshing out. It’s just something I post to show an example of a storyline to get players invested in the idea and connected to San’layn to want to help them and get them to join the Horde.
But yes, keep them as vampires. That’s one of the big reasons I like San’layn! 
