Step 1: Approach
Expand the squares of the sum and the difference.
Step 2: Sum and difference
Since the lines meet at $\vec a+\vec b$ (with both parameters equal to 1), $|\vec a|^2+|\vec b|^2+2\vec a\cdot\vec b=36$ and $|\vec a|^2+|\vec b|^2-2\vec a\cdot\vec b=16$.
Step 3: Subtract
$4\,\vec a\cdot\vec b=20$, so $\vec a\cdot\vec b=5$.
Step 4: Check
Adding gives $|\vec a|^2+|\vec b|^2=26$, a consistent positive value. So (A).
Final Answer:
The intersection point is a + b, so the squares of the sum and difference give a dot b = 5, option (A).
\[ \boxed{5} \]