Venting is often required at each fixture and each vent is then routed over to the associated sanitary riser. This is where things go awry. The fixture units calculation inside Revit can tolerate a closed system where the contents of the pipe and the air in the vent can't circulate via a loop (see later images). As soon as you create a loop condition fixture units cease to accrue to piping layout.
Most of the engineers I've met want to use a fitting that has an integral vent connection. The one you see in the images in this post was created by Scott Johnson and he shared it with me a couple years ago and it works nicely. Attempting to simulate this isn't a problem until you try to connect them all together.
The solution in the images here involves placing a cap at the top of each Vent riser to close the system. To complete the venting (that isn't really venting anymore) you just run additional piping above the cap elevation. The different appearance for the vent pipe and fittings is done with a filter. The vent pipe you see is a separate Pipe Type so simpler fittings can be used as it is sketched. A Type Comment parameter value of "Vent" makes it easy to use a Filter. In plan views I'd be inclined to hide the vent pipe using the same Filter so they don't compete with the sanitary pipe graphics, the fittings above tend to obscure the fittings below. If you place these caps as high as possible it might not be necessary to fuss with a filter. This image shows the System Inspector results after using this technique.
System Inspector won't even work until you create a situation that doesn't allow for this loop. By loop I mean this condition.
I've experimented with changing the nature of the connector in the wye fitting itself but unfortunately as soon as Revit detects an alternative path that creates a loop, no fixture units past that point.
I suppose you could argue, "What's the point? Since you can't use the sizing tool on sanitary pipe anyway?". It might not really matter to some, but if the pipe knows what fixture units are present it should help you do the sizing in-canvas. It will help because you can touch a pipe and see the value (in the Properties Palette). You can then refer back to your cheat sheets to choose the best diameter for the pipe.
Made a video too, watch at OpEd Videos or You Tube or listen and watch here (my graphics card goes haywire at the end a bit).