The best argument against allowing Sisters/Allied units not be able to use Vulkan's rule is the simplest, those units do not have the chapter tactic's rule. I forget who told me this but it was a good call I think it was Rich or Rick I apologize if it was some else
Again, sorry to continue the off topic, but I see a lot of arguing over this rule and I don't understand why.
Here's the rule in question: "All units in your army lose the Combat Tactics special rule. Instead, all Thunder Hammers in your army count as Master-Crafted, and all flamers, heavy flamers, meltaguns and multimeltas count as twin-linked"
The first and second half of Vulkan's rule have nothing to do with each other. Any unit that has Combat Tactics loses it. Then, any and all Meltas, Flamers and Thunder Hammers are improved - not just weapons used by units that had Combat Tactics, and not just Marines, either. Unlike the other characters, no exchange is being made. Whether a unit had combat tactics or not, in this case, is irrelevant.
Clearly, this was intended to twin link Salamander vehicles, but as written, I fail to see the argument that allies do not benefit from it.