When to use "wine vinegar" vs just wine?
Wine vinegar and wine are very different although are produced from the same thing. Wine vinegars are vinegars produced from fermenting wine by acetic acid bacteria which convert the ethanol in wine into acetic acid. 

Being a vinegar, it is much sharper than wine although like wine many flavours can be detected beyond the generic sharp lemon-like flavour, particularly in the premium aged varieties such as balsamic vinegar from Modena.

Wine vinegar has no alcoholic content in it and so there is no need to 'burn off the alcohol' which you would do when cooking with wine.

Wine has a much subtler flavour and so would use it in things like gravies, sauces, etc. Likewise, vinegar is much stronger and I would mainly use  it in vinaigrettes or in small quantities in cooking (ie finishing of a dish with a small drizzle of vinegar)in much the same way as you might use lemon.