I've asked "How could the sight tube not have a vacuum on it with the connection I've described?" and that question has not been answered and now I've created a "stir". This engine is Mercruiser original and this configuration has been discussed in this forum in the past. Unfortunately I've not connected with anyone that knows this engine. I wish I knew what the problem was. Maybe reading comprehension combined with a need to up your post count.

Well, I'm out.

Oh AllDodge, I have read that thread. It just doesn't help with my question.

Sorry to not be of help. I have the same engine setup you do with mechanical fuel pump and VST. So to answer the vacuum question, the line runs from the VST to the fuel pump. Under the plenum the line is teed and connects to a port which has about a 1/32 hole in the center of it. If the line was removed the hole is so small your engine wouldn't know it was off. The vacuum at the point should equal intake vacuum but since about anything can plug it up. I'm guessing here but since the hole is so small it could also take some time to draw enough vacuum to show up on a gauge, so you may or may not get a reading.

To answer your original question...

I've seen guys just lay the vent tubes from the mechanical pumps along where the intake and head meet and the gear lube still gets contaminated with fuel. So no matter where the vent tube is hooked up (VST tank for EFI, carb/flame arrestor on carb motors, or flopping around in the bilge) the pumps still weep fuel into the gear lube even though the pumps are otherwise "fine" and their diaphrams are not ruptured.