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For instance, “vous” has a silent “s”, unless it's something like “Je vous ai vu.” (I'm not sure that I constructed that properly, but you get the point — the s is pronounced in that case.)
But what if you have "Je voudrais acheter un billet."? Is it "voo-drez"?
Basically, I'm asking do you always pronounce it if the next word begins with a vowel, or only under certain circumstances?
A general rule: use whatever is the easiest to pronounce.
In the case of “Je vous ai vu”, I hope you will find [ʒə.vu.z‿e.vy] easier than [ʒə.vu.e.vy], because you would hardly be understood if you omit the liasion.
In “Je voudrais acheter un billet” it doesn't matter much. Personally I use a very light liaison (something between a /z/ and an aspirated /h/).
You will find more information on the Liaison (French) articles on Wikipedia.
I would say that usually when you have a s (or x) before a vowel, you can make the liaison. This is in order to avoid hiatuses. Usually, it's intended to be easier to pronounce. But if you don't pronounce it, you will be understood anyway, don't worry for that.
Full official rules from l'Académie française here. There are mandatory liaisons, forbidden ones, and optional cases (if you do the liaison then, it would be considered as “un parlé soutenu”, more or less the equivalent of the 'formal' register in English).
(Another link that may help, in French here, or this one in English.)
A quite common mistake on the liaison (confusion between a 't' and a 's' of sometimes a 'n' sound) is called “pataquès”.
I will add the particular case of nouns finishing by the letter "s" in their singular form: The classic rule says that if the "s" is pronounced in the singular form, then it is not pronounced in the plural form, so you must say "un os"[εnos] -> "des os"[dezo], "un ours"[εnurs] -> "des ours" [dezur], and that rule also explains why "moeurs" has to be pronounced [mɶr] and not [mɶrs].
s final au pluriel dans « des ours ». Une référence serait bienvenue.
Un comédien de la Comédie-Française et professeur au conservatoire, Georges le Roy (Traité pratique de la diction française, Grancher, 1968 [1932], p. 23), sans évoquer les règles qui régissent les accords, pour lesquelles il renvoie à sa Grammaire de la diction française (1912), propose quelques exercices de diction consacrés aux liaisons, qu'il introduit simplement ainsi (nous soulignons) :
Ne pas faire assez de liaisons est blâmable et quelquefois vulgaire ; en trop faire est prétentieux et quelquefois dangereux. L'important est de se souvenir que la règle principale en cette matière est l'harmonie.
Puis il poursuit par une distinction qui répond à la question tout en précisant cette exigence de sagacité ainsi, disons-le, de bon goût, que doivent travailler ces exercices, afin de
1o. Distinguer les liaisons qu'il faut faire et celles qu'il faut éviter ;
2o. S'habituer, sur des cas où la liaison est permise, à lier légèrement et avec facilité.
