How to perform measurement of a qubit?
You haven’t specified the order of entries in your vector. In general, the $i$’th entry is obtained with probability $|x_i|^2$.

Let’s consider two examples. In the first, the order of entries is $|00\rangle, |01\rangle, |10\rangle, |11\rangle$. The two final entries will be obtained with probability $1/2$ each, and so the first qubit will be measured as $|1\rangle$. If you measure only the first qubit, then you will still always get $|1\rangle$, and the other qubit will remain in the superposition $\frac{1}{\sqrt{2}}(|1\rangle-|0\rangle)$.

If the order of entries is instead $|00\rangle, |10\rangle, |01\rangle, |11\rangle$, then once again the two final entries will be obtained with probability $1/2$ each. This means that the first qubit will be measured as $|0\rangle$ or $|1\rangle$ with equal probability.