vennisa同vanessa邊個係真正既英文名寫法?

Vanessa should be the right one.
However, some people may make a name on their own. it's simply because they like to be unique and special. and some even make an English name that sounds like the pronounciation of their chinese name.
dont care so much whether you can find the names on the dictionary or not. use whatever name that suits you much is fine =)

The correct spelling is Vanessa...冇人用Vennisa

I think Vanessa...
My friends are usually use Vanessa..