It doesn't mean signs of how frisky they were; it means signs that show whether the animal was alive when the lesions occurred or whether they were postmortem. This is about examining lesions on dead animals.

"Diferenciación macroscópica entre las lesiones vitales y postmortales. [...]
En ocasiones puede no presentarse los signos de vitalidad aun cuando la lesión se haya producido en vida y también lo contrario. [...]
Prueba del lavado. Cuando existen dudas acerca de la vitalidad de una herida se puede recurrir a la prueba del lavado mediante la aplicación de un chorro fino de agua sobre la sangre coagulada. Si la arrastra la herida será postmortal, en caso contrario será vital.2
http://webs.ucm.es/centros/cont/descargas/documento30332.pdf

This is about wounds inflicted by, not on, an animal, but the principle is the same:

"Therefore, signs of vitality on wound site, presence of bleeding, and results of histological examinations to be performed are very important in discrimination between antemortem and postmortem dog attack"
https://www.researchgate.net/publication/248157892_Postmorte...

And if you look for "signs of vitality" in the context of postmortems you'll see that it's the expression for signs that enable you to determine that things occurred before death (such as soot in the lungs of people who died in fires).


--------------------------------------------------
Note added at 2 hrs (2018-05-01 13:02:48 GMT)
--------------------------------------------------

I suppose it's possible that it refers to the pig itself, in which case it would means signs that a pig is alive when it looks as though it might be dead; the first meaning of "vitalidad" is "cualidad de tener vida" (DLE): the state of being alive. But "signs of vitality" would still be right; as a medical term, vitality similarly means "the condition of being alive" ( http://medical-dictionary.thefreedictionary.com/vitality ). Surely not vital signs (body temperature, blood pressure, pulse and breathing rate), which are called "signos vitales".

--------------------------------------------------
Note added at 1 day 8 hrs (2018-05-02 18:10:30 GMT) Post-grading
--------------------------------------------------

Nice of you to say so, Neil, but more by luck than judgement this time, really; I didn't realise it meant what the client says.