From "Livestrong.com" --One cup of sliced cucumbers, with the skin on, also contain 223.60 IU of vitamin A, which is 4.5 percent of the RDA, and 5.5mg of vitamin C which is almost 10 percent of the RDA. Other trace vitamins in cucumbers are niacin and folate (both from the B vitamin family), B6, vitamin E and vitamin K.
"Trace" means there is not very much of them, so that kind of disproves the "good source of B vitamins" thing.
They are low in calories and high in fiber. However to quote Fred Sanford, cucumbers "make me burp."