Yes you can have too much vacuum in the crankcase, because seals can only withstand a given pressure difference. If you have a seal failure due to an excessive vacuum in the crankcase you might suck contaminants into your crankcase. With the engine off and no vacuum, a failed seal can let some oil seep out, which will attract dust and other contaminants, and then when the engine is started again this dirty oil gunk is sucked back into the crankcase!

Based on my crankcase pressure measurements last year with the stock CCV coupler in place, I saw a maximum of about a 7" H20 vacuum when towing the steepest grade from Smokin to Asheville, and I think that would be a safe level of vacuum to apply to the crankcase.

BTW, the stock CCV coupler looks somewhat like the bottom pic in post #41 here, except that it has a raised portion where the vent tube that connects to the doghouse is located. This raised portion forms a "Venturi" which increases the flow velocity in the region where the vent tube is located, and according to the "Bernoulli effect" there is a lower static pressure in a region where the flow velocity is increased.

So why does the stock CCV coupler, which employs a bung connected with a hose to the crankcase provide a vacuum, while a somewhat similar bung in the tailpipe with a hose to the crankcase provide an above ambient pressure? It's because of relativity, but not the Einstein type.

The input to the bung in the stock CCV coupler is connected to ambient pressure via the air filter, and the output end is connected to the turbo input which is a source of less than ambient pressure. Now, the reduced static pressure in the CCV coupler bung due to the "Bernoulli effect" is relative to the input flow, which is at ambient, so the pressure in the bung is somewhat less than the input ambient pressure, which by definition means that it's a vacuum.

In the tailpipe configuration, the input to the bung is connected to an above ambient pressure from the turbo output, and although the exhaust flow causes a reduced static pressure in the bung due to the "Bernoulli effect", it's a reduction relative to the input pressure. Typically, the input is at about 30" H2O above ambient, and the tailpipe bung only see's about a 3" H2O reduction (because there's not a "Venturi" in the tailpipe like in the stock CCV coupler), and a 30" H2O input less a 3" H2O decrease still leaves a 27" H2O or about a 1 psi above ambient pressure in the bung, and at WOT this is the typical pressure that's measured in a crankcase vented to a bung in the tailpipe!

Well, I think I've shot my wad on this topic, as I don't think I can come up with a more clearly stated explanation of why a bung in a tailpipe won't produce a vacuum, unless I resort to a swarm of bees flying down the tailpipe, and I already used that one to explain the static pressure reduction that Tenn measured in his 6637 test.