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You can simply iterate over your list of vectors and search which vector gives a zero cross product with your target vector:

def search(L, target):
    for i,v in enumerate(L):
        if v.cross_product(target) == 0:
            return i
    raise ValueError('No vector found')

L = [vector([1,0,0]), vector([0,1,0]), vector([0,0,1])]
v= vector([6,0,0])

search(L, v)
0

search(L[1:], v)
ValueError: No vector found