Answer
Vector $\textbf R$ and vector $ \textbf A$ must be parallel
Work Step by Step
We have:
$V=|\textbf R\cdot \textbf A|= |\textbf R ||\textbf A| \cos\theta $
For $V$ to be maximum, $\cos\theta$ must have its maximum value $\cos\theta=1$, therefore $\theta$=0.
This means that the vectors $\textbf R$ and $ \textbf A$ must be parallel to collect the maximum amount of water
