You still should vorlauf prior to each runoff. The stirring which is absolutely required, will usually jam some grain bits through your separation filter. It's not really critical, but it doesn't hurt either.
The process of batch sparging immerses a mass of grain that is covered with sugar into a volume of water. The process called diffusion takes over and the sugar content of the whole system becomes equalized and homogenous across the entire mixture. You drain that wort out and there is no chance for channeling.
Channeling is relevant in fly sparging and can only be fully appreciated if you can picture how fly sparging works. The entire mass of grain and mash wort starts out homogenous across the entire volume (just like batch sparging right before the runoffs). You start adding water on top and draining at the same time to slowly create a sugar gradient, lowest SG on top, highest SG on bottom. You still get diffusion, but a high sugar concentration delta is maintained. Channeling is where that lower SG water/wort on the top finds a path that is less resistant than straight down. That neat horizontal gradient gets disturbed and you leave sugar behind.
