Xenology could gate the attempt to use these skills, it could add blue or black dice, it could change the percentage of profits/the wounds healed/etc. There are any number of things it could do, but if it's vital, then it should be doing something to change the given numbers, whether that change is in the dice, the tally, or the final result.

Once I am told that negotiation adds a 5% profit per success, I want to know how a "vital" xenology roll affects that. If the negotiation results had been left entirely up to the GM, with no systemic guidelines, then I wouldn't find xenology being the same 'left to the GM' as odd.

IMO, this is one of those cases where you do just have to leave it to the GM.

With my GM, I believe that he would modify the difficulty of your Medicine check based on how "alien" the recipient is, and if you want to make that easier then you want to have a good Xenology check. So, success on a Xenology check wouldn’t give you [boost] dice and failure wouldn’t give you [setback] dice. But if you are successful then the Medicine check might be just Hard instead of Formidable or worse.

Of course, the irony here is that Far Horizons does talk about how Xenology works for doctors.

Pages 66 and 67 go into some depth about how to run doctors and make them fit into the campaign, etc…. The last paragraph on page 67 specifically addresses Xenology:

For example, when treating a species unfamiliar, or at least less familiar to the Doctor, the PC might make a Knowledge (Xenology) check for insight into the proper treatment. Success might upgrade the ability of the Doctor’s Medicine skill for one roll, whereas failure might upgrade the difficulty of the roll. [Advantage] or [Threat] might add [boost] or [setback], respectively. A successful check might also allow the Doctor to attempt to heal one Critical Injury and one wound for each [success].