If it says +9, then it's +9. If it says 9%, then it's 9%.

So, for instance, many potions show percents in their description...and work as percents.

Then you have armor that likes to give +30 vitality.

Well, that does give +30 vitality if you look at the effect of putting it on and taking it off.

So, it's fair to say +9 and 9% can both be found in an item description, and both mean exactly what they say.

What I mean is, is there a way to gauge how much +1 vitality or +1 armor really is? How much of an effect does +9 vitality have on your health? Is +9 a lot or a little? Reason I'm asking is because there are some skills that add vitality, sign effect, and damage but because I don't know how well the numbers work in real practice, I have trouble deciding which skills are sufficient for my needs.

You can view a summary of your character's stats from the character screen (press Enter). There you will find the exact values, so you'll be able to decide whether +9 vitality is a lot or a little when you see your total.

Generally, 1 armor will prevent 1 damage from every hit (I'm not sure about magic), and 1 damage will temporarily reduce your vitality by 1 (if it reaches 0, you die).