The Vorlauf fitting was designed to temporary recirculate the wort towards the end of the mash rest to clarify the first runnings from residual grain flour or mill finings. This fitting was designed primarily to replace a pitcher or any other small container that most guys use to clear their first runnings. Essentially, you would recirculate with a pump for 5-7 mins and end up with ultra-clear wort.
On the other hand, the manifold was designed for users that plan on using their Infussion Series Mash Tun within a more complex recirculation mash system such as a RIMS or HERMS system. It is ideal for this purpose because the manifold will evenly distribute heated wort across the grain bed. This is important so that the temperature within the vessel remains uniform, especially in the midst of a multi-step mash regimen or aggressive temperature ramp.