Hi Daniel,
Yes, you can use vectors in the solve block. See my attached worksheet. Note a couple of things:
1. You need to put guess values for Fs and C, since they are not defined elsewhere in the worksheet and their values will depend upon c.
2. Your last constraint shows sum(Fs)+C=0. If I understand your initial question, you are only looking for the sum of the vectors to equal a zero vector. As you now have the constraint, the solve block will take the sum of the values in Fs (resulting in a scalar value) and then add that to each value in the vector C. I don't think that's what you want. So I removed the summation operator.
Hopefully that helps!
Luke