There were actually grapes, http://en.wikipedia.org/wiki/Vitis_riparia specifically!

But the most prevalent fruit were Currant berries (black and red), which makes very tasty wine! Beyond this any other fruits and berries they found were used, as well as aromatic flowers like elderflower, onion, dandelion, etc.

...they likely tried to ferment anything they found.

As an aside... The name Vinland has two possible meanings, one being related to vines/grapes, and the other suggestion involves interpreting the name not as vín-land but as vin-land, with a short vowel. Old Norse vin has a meaning of "meadow, pasture". This interpretation of Vinland as "pasture-land" rather than "vine-land".

The vikings were mead makers and later brewers but not wine growers. The "vin" would reference the vines they found there not wine itself. Even if the vikings had a wine making and drinking culture, North America would not have been the place to do it at the time. The north american native varieties of grapes are for the most part not suitable for winemaking and if they did in theory try planting "wine grape" varieties imported from Europe the vines would have died from native pests almost immediately.