DWV Layout Basement Bathroom - layout review

[great white]

We are in the planning stages of adding bathroom to basement. This new bathroom is directly below existing upstairs bathroom. House is 1 story ranch. Looking for additional opinions. Most of the plumbing contractors are saying, AAV vent for vanity and no other vents required do to close proximity to existing stack. I am also concerned about the toilet drain connecting upstream of shower and vanity. Is there a risk with flow of toilet siphoning out the traps for shower and vanity?

[cacher_chick]

The existing stack has no effect on the proposed addition.

If that is what they are telling you they are apparently are not versed in proper plumbing or the plumbing code.

Michigan uses the International Plumbing Code (IPC)

[Terry]

You can not use the upstairs waste stack for the downstairs bath as any sort of vent. It's the waste stack for the upstairs.
Wet venting is only for a bathroom group on the "same" floor. That means you can using wet venting "within" the bathroom group (altogether on one floor)

Your layout does not vent the shower,
The lav would vent by the AAV