"Let's drink more wine."
Translation:Bebamos más vino.
I believe that one exception to the rule that the present subjunctive is used with the "nosotros" command form is "ir" - vamos a + infinitive can be used for let's.....In the above case it would be okay as mimroma suggests. In addition, I understand that "vamos" can mean let's go (the negative would, however, be "no vayamos" - let's not go).
Yes that is correct, however, I think it only works when you mean "let's go/leave" then you would use "vamos", if you want the first person command "let's + (verb)", then you need the "nosotros" form in the subjunctive. Here's a link about it: http://www.studyspanish.com/lessons/noscomm.htm
That's in the future. We're going to drink more wine. It expresses certainty about the fact that it will happen in the future. This is present tense subjunctive, and I can't explain subjunctive perfectly because it still rattles my brain, but essentially it refers to conditions that do not yet exist for certain (i.e. in this case, "us drinking more wine" which is why I give the suggestion)