I was wondering if "vector" is countable or not. I've been using it in plural form (vectors) but I don't know if that is correct.
Yes, vector is countable, and the plural form is indeed vectors as mentioned in the Merriam-Webster Learner's Dictionary:
vector /ˈvɛktɚ/ noun
1 mathematics : a quantity (such as velocity) that has size and direction
If you search for "vectors" on Google Scolar you will also find hundreds of thousands of scientific publications using the plural form, so I would say it is definitely accepted to say vectors.
To tell weather or not a noun is countable, you refer to a dictionary.
Here's what LDOCE says:
- a quantity such as force that has a direction as well as size
- an insect or animal that passes disease from one person to another [= carrier]:
Mosquitoes are feared as vectors of malaria.
- in biology, an animal or human cell that is used to carry DNA from one cell to another to produce a clone