Not entirely sure what you are asking.
An open vector is like a straight line or a curved line. The ends are NOT connected - or closed. That is an example of an open vector.
On bit map tracings a bitmap line that is traced will often result in two parallel lines, but they would be closed. Of course there are exceptions. If I had a straight line - BITMAP - and I automatically traced it I would then have a closed vector, but that vector would be a double line that traced the outside of the bitmap line.
Soo - do you really - want a open vector? For that, I personally would do it manually, not auto trace.
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