Dominic,
1."Да вы пьяны!" - You are drunk!
Here 'вы' is used to refer to a single person. It is polite 'вы'. Пьян - is a short form of adjective (the one used in predicative function).
And here it stands in plural, as you expected! It is a rule for short forms of the adjectives and participles: Вы устали (you are tired) etc.
2. Вы красный!
As you have, most probably, noticed both short and full forms can be used as predicates sometimes. It depends on the particular word used, and sometimes on the meaning...
Full adjective would normally stand in sungular here:
Вы (polite) красный, вы (a group) красные.
I'm not sure, how did it work in 19 century... 'polite plural' is not something born out of the grammar in a natural way) Rules might change here.
She could say 'вы красны!" as well, but 'красный' is more common here, while 'вы пьяный' is unlikely.
3. a short form would often mean a property/atribute of an object, often temporary. A full form as a predicate may mean the same - or to act as a substantivated adjective, i.e. as a noun. Dovlatov uses it this way:)
Cf. вы - хороший человек!
человек is a single person, so хороший is in singular.
BTW, that's how '*вы пьяный' could be understood. We use пьяный to mean 'a drunk man' too often.
May be that's related to the fact, that 'full forms' has developed form 'a short form + demonstrative pronoun'. Хорошая девочка/ хороший мальчик /орошие дети is etymologically 'that good girl/boy', 'those good children'.