Can I use sugar in vegetarian cooking?
Some "plain old table sugar" is filtered through charcoal. And charcoal is sometimes derived from animal bone. The sugar in that bag at the store might come from sugar beets, which don't need filtering, or from sugar cane, which does. But you usually can't tell which by reading the bag.
That means that although the sugar itself is not an animal product, it may be filtered through charred animal bone before it goes in the bag. And although the animal bone in question has been altered to such a degree that even strict followers of the kosher laws allow it, vegetarianism and kosher Judaism are two completely different things. So I don't recommend using the kosher excuse with your vegetarian guests.
It's best to discuss the "plain old sugar" issue with your guests. If they admit to using the stuff at home, you're off the hook! But if they are adamantly opposed to the use of regular sugar, use evaporated cane juice crystals (unfiltered sugar). These are available in health food stores and better supermarkets under brand names such as Hain, Sucanat, and Florida Crystals.
If these are not available where you live, you can substitute maple syrup or corn syrup. These are vegan by everybody's definition. (Yes, corn syrup might be produced by an industrial process, but it's not filtered through animal bones. One issue at a time, please.)
Keep in mind that using syrup instead of sugar adds a bit more liquid to your recipe. Reduce other liquids to compensate, so that the result isn't too runny.
As for confectioner's sugar (usually used in frostings), this is finely ground white sugar. you're a bit out of luck there unless you can be certain that the sugar comes from sugar beets. If your guests are picky about the sugar-and-charcoal issue, use alternative toppings - a jar of blackberry jam (usually sweetened with corn syrup, but check the ingredients) makes a swell frosting!
Honey, as we all know, is produced by domesticated honeybees. It is reasonable to draw an analogy to milk production. So most vegetarians will certainly eat honey. But vegans strive to consume no animal products at all. So there is a good chance they will object to the use of honey. Talk to your vegan guests about the honey issue. And if you don't have that option, don't use honey. Real maple syrup is a good alternative.
I won't describe honey as vegan - it's an animal product, it's not vegan. However, some vegans do eat it anyway. You can't go wrong by asking. Well, maybe you can! But in the long run even the most difficult vegan guest will appreciate that you went the extra mile by consulting them.