In theory, the Russian language, unlike, say, English, allows any word order, so both sentences are grammatical, as well as "Вас мы видим," "Видим мы вас" and so on. This is not to say that all of these are interchangeable. Neither are they equally common. Word order depends on the context, on what it is exactly that you're trying to say. You can see every Russian sentence as having a) the theme, which is something you already know and which serves as a starting point, and b) the new information (i.e. what you have to say on the theme). Generally, the theme tends to be at the beginning of the sentence, while the new information tends to be at the end thereof. Let's analyze the two sentences.
1) Мы вас видим. Here, "мы вас" is the starting point, and what we have to say on that is "видим." One interpretation might be that the speaker wants to say this: "Oh, you guys think that we can't see you, don't you? Yet we do see you guys."
2) Мы видим вас. Here, "мы видим" is the theme, and the new information is "вас." You can paraphrase this as "It is you guys that we see."
So, choosing another word order can slightly change the meaning of the sentence, or rather what you want to stress in the sentence.
This distinction is more important in written Russian. Choosing the correct word order depending on what you're actually trying to say makes your texts more coherent.
However, this is not as important in colloqual Russian because a) colloqual speech is spontaneous, and b) you can stress the most important word with intonation.
I hope this helps