I'm not sure it's commonly used anywhere anymore...it was certainly in extensive use in the UK through the 19th century. OED currently lists it as "dated" rather than "archaic," so possibly still some use there. There's plenty of evidence for it in common literature (i.e. Stephenson's "Treasure Island:" "...bring up the cold pie and let him sup."). Each meal in English has a dedicated verb to go with it: "I breakfast/he breakfasts," "I lunch/he lunches," "I dine/he dines," "I sup/he sups," "I snack/he snacks." Even tea managed to get one, despite being a relatively recent innovation and an awkward construction ("I tea/he teas"), though I dare say it's rarer than "sup."
vespermangxo is the noun form, like "Dinner is on the table" (Vespermangxo estas sur la tablo), while the other two the verb forms (we say "to eat dinner", but they have one word for it). Vespermangxi is the inifinitve form, and vespermangxas is the present tense form as in "I eat dinner" (Mi veserpermangxas).
Also, in some parts of the U.S., "dinner" is the noontime meal and "supper" is the evening meal but, even in places where "dinner" is the evening meal, "supper" is still understood as the evening meal, as well. Likewise, even though "dinner" can be ambiguous, "lunch" is always understood for the noontime meal.
It was the same in Australia some years ago. Breakfast, dinner & tea. Supper was a light meal or snack later in the evening. "Lunch" has now largely replaced dinner for the midday meal and dinner has been moved to the evening meal. The post-tea/dinner cry was always "What's for pudding?", meaning "What dish is about to be presented as dessert".
Ireland is the same. My wife and I confuse our kids with the mid-day meal. She calls it dinner and I call it lunch. Often it's an urban/rural divide. Dinner generally refers to the largest meal of the day, in rural areas this is generally the midday meal and in urban areas it tends to be the late meal.
In England, breakfast is always at the start of the day, and lunch is always sometime around midday. After that, it is not even down to regional differences. It varies from family to family.
The midday meal: Lunch or dinner.
The mid-afternoon snack, with a cup of tea or coffee: Tea.
The meal served at the end of the afternoon or at the beginning of the day: Dinner or tea or supper.
A snack before bedtime or late evening: Supper.
I wrote "Do they eat supper together" and I was wrong. I guess in Esperanto Land, supper is the late evening snack!