So, I've been asking various people for some time, and decided to post this short list here. The goal is to avoid compatibility issues and claim unused values in an orderly fashion.

I encourage you to post comments with new info, whenever you learn about a new index integer value being used for a vampire mod!
This idea has been approved by TR and SHOTN developers on Discord, so, fellow vampire enthusiasts, don't hesitate to add new content to vampires of Morrowind, work with all publicly available Masterfiles and best of luck with your ideas! I personally can't wait to see OperatorJack's Vampyr mod blossom into something extraordinary. 
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VampClan Index:

-1 - cured
0 - not in any clan, default value
1 - Aundae
2 - Berne
3 - Quarra
100 - Baluath (Tamriel Data)
101 - Orlukh (Tamriel Data)
102 - Draneth (Tamriel Data)
103 - Ehad (Tamriel Data)
104 - Rudhagor (Tamriel Data)
200 - Khulari (Tamriel Data)
616 - Ferver (QCVL)
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Disease ID:

"Vampire Blood Aundae"
"Vampire Blood Berne"
"Vampire Blood Quarra"
"T_Vamp_Dis_Baluath"
"T_Vamp_Dis_Orlukh"
"T_Vamp_Dis_Draneth"
"T_Vamp_Dis_Ehad"
"T_Vamp_Dis_Rudhagor"
"T_Vamp_Dis_Khulari"
"Vampire Blood Ferver"
...
Update: as of 12.12.2022, there is a new TR wiki page covering all this info with more detail:
https://wiki.project-tamriel.com/wiki/Vampirism